Docs/SourceKit: add some documentation for expression type request.

Author: nkcsgexi

tools/SourceKit/docs/Protocol.md

@@ -737,7 +737,46 @@ Welcome to SourceKit.  Type ':help' for assistance.
 }
 ```
 
+## Expression Type
+This request collects the types of all expressions in a source file after type checking.
+To fulfill this task, the client must provide the path to the Swift source file under
+type checking and the necessary compiler arguments to help resolve all dependencies.
 
+### Request
+
+```
+{
+    <key.request>:            (UID)     <source.request.expression.type>,
+    <key.sourcefile>:         (string)  // Absolute path to the file.
+    <key.compilerargs>:       [string*] // Array of zero or more strings for the compiler arguments,
+                                        // e.g ["-sdk", "/path/to/sdk"]. If key.sourcefile is provided,
+                                        // these must include the path to that file.
+}
+```
+
+### Response
+```
+{
+    <key.printedtypebuffer>:          (string)                    // A text buffer where all expression types are printed to.
+    <key.expression_type_list>:       (array) [expr-type-info*]   // A list of expression and type
+}
+```
+
+```
+expr-type-info ::=
+{
+  <key.expression_offset>:    (int64)    // Offset of an expression in the source file
+  <key.expression_length>:    (int64)    // Length of an expression in the source file
+  <key.type_offset>:          (int64)    // Offset of the printed type of the expression in the printed type buffer
+  <key.type_length>:          (int64)    // Length of the printed type of the expression in the printed type buffer
+}
+```
+
+### Testing
+
+```
+$ sourcekitd-test -req=collect-type /path/to/file.swift -- /path/to/file.swift
+```
 
 # UIDs