Docs: Slight tweak to description of the alias metaparameter

- Delete the link to the old language guide; there's no "more information" in there.
- Explain effects in practical terms instead of expecting the reader to know the
  diff between the parser and the "library."
- Adjust quoting.
- Fix technically-correct-but-not-what-we-were-trying-to-show syntax.
Merge pull request #1822 from nfagerlund/alias_metaparameter

Docs: Slight tweak to description of the alias metaparameter
Add debug_once to util/warnings

This function provides debounced logging for `Puppet.debug` similar to
`notice_once` and `warnonce`.
(#21835) Downgrade missing type optimization warn

For some catalogs, enabling `ignoremissingtypes` can produce a lot of noise in
the logs. This change downgrades the warning to a debug message and debounces
the logger so that messages appear only once per missing class or definition.