Fix swapped charCodeAt/codePointAt JSDoc descriptions

[Shoaib Khan (techreign)]

What was wrong

String.prototype.charCodeAt's JSDoc said it returns the "Unicode value" of
the character, which is ambiguous and actually describes what
codePointAt does. Meanwhile codePointAt's JSDoc led with "the code
point value of the UTF-16 encoded code point", which buries the fact that
it returns the full decoded Unicode code point (surrogate pairs handled),
the very thing that distinguishes it from charCodeAt. Per MDN:

• charCodeAt() returns the UTF-16 code unit at the given index.
• codePointAt() returns the Unicode code point value at the given
  position (decoding a surrogate pair if one starts there).

The repo's src/lib/*.d.ts paths from the original issue have since moved
under the Go-port restructuring; the current source of truth for these
declarations is tsc/internal/bundled/libs/lib.es5.d.ts (charCodeAt) and
tsc/internal/bundled/libs/lib.es2015.core.d.ts (codePointAt). The
hereby lib task only copies these files into built/local, it does not
generate them, so they're the correct place to fix.

What changed

• charCodeAt: doc now says it returns the UTF-16 code unit at the index.
• codePointAt: doc now leads with "Unicode code point value" instead of
  "UTF-16 encoded code point", and keeps the existing surrogate-pair detail
  (undefined semantics preserved, no behavior/signature change).

Doc comments only, no code or signature changes.

Fixes #49561

How it was verified

npm ci
npx hereby lib
git diff --stat

• npm ci completed cleanly.
• npx hereby lib ran successfully and copied the updated declaration
  files into built/local, confirming the fix propagates through the
  repo's lib-copy task without errors.
• Confirmed via git diff that only the two doc comment blocks changed
  (no generated output is committed; built/ is gitignored).
• tsc/internal/bundled/libs/** is excluded from dprint fmt/dprint check by .dprint.jsonc, and hereby lint runs golangci-lint over the
  Go modules only, so neither tool applies to these .d.ts files; I did
  not run the full Go build/test suite per the task scope (docs-only
  change, no compiler behavior affected).

Disclosure

This PR was authored with Claude Code, operated by me (the account owner
who forked and opened this PR). I read and reviewed the diff before
opening this PR, chose this specific issue deliberately, and will handle
CLA acceptance and any review feedback myself.

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