gh-97607: Fix content parsing in the impl-detail reST directive (#97652)

* Don't parse content as arg in the impl-detail directive

This does not change the (untranslated) output,
but ensures that the doctree node metadata is correct.
which fixes gh-97607 with the text not being translated.
It also simplifies the code and logic
and makes it consistant with the docutils built-in directives.

* Remove unused branch from impl-detail directive handling no-content case

This is not used anywhere in the docs and lacks a clear use case,
and is more likely a mistake which is now flagged at build time.
This simplifies the logic from two code paths to one,
and makes the behavior consistant with similar built-in directives
(e.g. the various admonition types).

* Further simplify impl-detail reST directive code