Apply the reviewed tooltip.h comment wording from the branch-target
sequential review run.

What changed:
- updated the Show method header comment to describe the actual visible
  behavior more directly
- clarified that considerCursor moves the tooltip below the cursor
- clarified that modal mode waits for messages that close the tooltip and
  returns after the tooltip is hidden

Why it changed:
- the reviewed wording is clearer and matches the current implementation in
  tooltip.cpp without changing code behavior
- the branch-target artifacts for src/tooltip.h passed comment guard, so the
  change remains comment-only and safe to land

How the result was verified:
- inspected the reviewed patch and the tooltip implementation for semantic fit
- regenerated the triple diff HTML for the run
- ran comment guard on the delivery branch checkout and confirmed zero
  violations in strict and ignore-whitespace modes
- checked git diff and git diff --check for src/tooltip.h before committing