Convert the formal grammar for DocC compatibility (#55)

DocC doesn't support subscripts, so instead of using subscript "opt" for
optionality, use a postfix question mark. Alternatives considered
included text like "OPT" or "(opt)" and Unicode subscript characters.
The former are hard to read and interrupt the flow of the grammar. The
latter render poorly because they're intended for phonetic annotation,
so the different letters don't share a baseline in many fonts.

It's a known issue that DocC doesn't support hard breaks [1], so insert a
paragraph break between each grammar production for now. I added a
"double" paragraph break between groups, where there used to be a blank
line, to preserve that information for when DocC adds hard break support
in the future.

1: https://github.com/apple/swift-docc/issues/412

Fixes #3

Fixes rdar://101001280

Author: amartini51

Sources/TSPL/TSPL.docc/ReferenceManual/AboutTheLanguageReference.md

@@ -22,43 +22,34 @@ follows a few conventions:
 - An arrow (→) is used to mark grammar productions and can be read as "can consist of."
 - Syntactic categories are indicated by *italic* text and appear on both sides
   of a grammar production rule.
-- Literal words and punctuation are indicated by boldface `constant width` text
+- Literal words and punctuation are indicated by **`boldface constant width`** text
   and appear only on the right-hand side of a grammar production rule.
 - Alternative grammar productions are separated by vertical
   bars (|). When alternative productions are too long to read easily,
   they're broken into multiple grammar production rules on new lines.
 - In a few cases, regular font text is used to describe the right-hand side
   of a grammar production rule.
 - Optional syntactic categories and literals are marked by a trailing
-  subscript, *opt*.
+  question mark, *?*.
 
 As an example, the grammar of a getter-setter block is defined as follows:
 
-```
-Grammar of a getter-setter block
-
-getter-setter-block --> ``{`` getter-clause setter-clause-OPT ``}`` | ``{`` setter-clause getter-clause ``}``
-```
-
+> Grammar of a getter-setter block:
+>
+> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`** | **`{`** *setter-clause* *getter-clause* **`}`**
 
 This definition indicates that a getter-setter block can consist of a getter clause
 followed by an optional setter clause, enclosed in braces,
 *or* a setter clause followed by a getter clause, enclosed in braces.
 The grammar production above is equivalent to the following two productions,
 where the alternatives are spelled out explicitly:
 
-```
-Grammar of a getter-setter block
-
-getter-setter-block --> ``{`` getter-clause setter-clause-OPT ``}``
-getter-setter-block --> ``{`` setter-clause getter-clause ``}``
-```
-
-
-*getter-setter-block* → `{` *getter-clause* *setter-clause?* `}`
-
-*getter-setter-block* → `{` *setter-clause* *getter-clause* `}`
-
+> Grammar of a getter-setter block:
+>
+>
+> *getter-setter-block* → **`{`** *getter-clause* *setter-clause*_?_ **`}`**
+>
+> *getter-setter-block* → **`{`** *setter-clause* *getter-clause* **`}`**
 
 <!--
 This source file is part of the Swift.org open source project

Sources/TSPL/TSPL.docc/ReferenceManual/Attributes.md

@@ -2237,21 +2237,30 @@ at the time the code is compiled.
 For an example of how to use the `unknown` attribute,
 see <doc:Statements#Switching-Over-Future-Enumeration-Cases>.
 
-```
-Grammar of an attribute
-
-attribute --> ``@`` attribute-name attribute-argument-clause-OPT
-attribute-name --> identifier
-attribute-argument-clause --> ``(`` balanced-tokens-OPT ``)``
-attributes --> attribute attributes-OPT
-
-balanced-tokens --> balanced-token balanced-tokens-OPT
-balanced-token --> ``(`` balanced-tokens-OPT ``)``
-balanced-token --> ``[`` balanced-tokens-OPT ``]``
-balanced-token --> ``{`` balanced-tokens-OPT ``}``
-balanced-token --> Any identifier, keyword, literal, or operator
-balanced-token --> Any punctuation except ``(``, ``)``, ``[``, ``]``, ``{``, or ``}``
-```
+> Grammar of an attribute:
+>
+> *attribute* → **`@`** *attribute-name* *attribute-argument-clause*_?_
+>
+> *attribute-name* → *identifier*
+>
+> *attribute-argument-clause* → **`(`** *balanced-tokens*_?_ **`)`**
+>
+> *attributes* → *attribute* *attributes*_?_
+>
+>
+>
+> *balanced-tokens* → *balanced-token* *balanced-tokens*_?_
+>
+> *balanced-token* → **`(`** *balanced-tokens*_?_ **`)`**
+>
+> *balanced-token* → **`[`** *balanced-tokens*_?_ **`]`**
+>
+> *balanced-token* → **`{`** *balanced-tokens*_?_ **`}`**
+>
+> *balanced-token* → Any identifier, keyword, literal, or operator
+>
+> *balanced-token* → Any punctuation except  **`(`**,  **`)`**,  **`[`**,  **`]`**,  **`{`**, or  **`}`**
+