Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/2069
Fix documentation inconsistencies in absl/strings/escaping.h:
- Add missing `true` return value documentation to
WebSafeBase64Unescape(), aligning it with the parallel
Base64Unescape() which already documents this.
- Fix missing closing backtick in WebSafeBase64Escape() comment
(`dest` was unterminated).
- Fix inconsistent single quotes to backticks on `src`/`dest`
references, matching the style used elsewhere in the file.
Merge 0616be3080 into fe126c727b
Merging this change closes#2069
COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/2069 from DrishtiTripathi2230:docs/websafe-base64-unescape-return-value 0616be3080
PiperOrigin-RevId: 924786942
Change-Id: Idcd6d27e1da696614cd8c9417bb6facb277d1d47