Document charAt edge case behavior

[bwalter007]

Similar to PR #63344, this clarifies what happens when charAt is called with an out-of-bounds index (returns empty string).

Improves documentation consistency across String methods.

[afurm]

Good clarification — charAt returning empty string for out-of-bounds indices is per the ECMAScript spec, and this docs update correctly reflects that.

Question: do charCodeAt and codePointAt have the same out-of-bounds behavior documented in their JSDoc? It might be worth checking if a parallel doc update is needed for consistency across the string index methods. Also, does substring document its negative-index handling (which maps to 0)?