docs(Tooltip,Menu): clarify offset interaction with screen-edge clamping

[arbaev]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Documentation
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the dev branch
• When resolving a specific issue, it's referenced in the PR's title
• It's been tested on a Cordova (iOS, Android) app
• It's been tested on an Electron app
• Any necessary documentation has been added or updated, or explained in the PR's description

Other information:

Follow-up to #18212 (working-as-intended, not an offset bug).

The horizontal/vertical offset on QTooltip/QMenu can appear to have no effect.
It isn't an offset bug: offset is applied to the anchor element's bounding box
and the result is then clamped to the viewport, so anchoring to a full-width /
screen-edge element (or pointing anchor/self toward an edge with no room)
masks the offset on that axis.

This adds a tip to the Positioning section of both the Tooltip and Menu docs
explaining the behaviour and how to avoid it.

[rstoenescu]

Thanks for contributing!