gh-62172: Document findsource() and getblock() in inspect

[sreehariannam]

Continues gh-62172 (see #153624 for the first batch: classify_class_attrs, getabsfile, indentsize). This adds documentation for two more public, __all__-exported inspect functions that were missing from Doc/library/inspect.rst:

• inspect.findsource()
• inspect.getblock()

There was an earlier attempt at documenting findsource in #6992, which a core reviewer approved with a minor suggestion (mention that findsource doesn't unwrap decorated callables like getsourcelines does) before it went stale and was closed. That suggestion is incorporated here.

Remaining undocumented functions from gh-62172 (walktree, getargs, formatannotation, formatannotationrelativeto, getlineno) are left for follow-up PRs to keep this reviewable.

Verified with sphinx-lint (no issues).

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33555136 | 📁 Comparing d5f01a8 against main (832bc0b)

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2 files changed

± library/inspect.html
± whatsnew/changelog.html