bash-completion2: add description, clean up
bash-completion2: add description, clean up

Migrate inline patch to `formula-patches` repository, remove `compdir`
method (no longer needed), remove broken `post_install` method (broken
since core/formula repository split), and trim caveats (stop referencing `bash-completion` formula unnecessarily).

Closes #1286.

Signed-off-by: Martin Afanasjew <54669547a225ff20cba8b75a4adca540eef25858@afanasjew.de>

heuermh/homebrew-versions,Ashton-W/homebrew-versions,DomT4/homebrew-versions,coeligena/homebrew-verscustomized,UniqMartin/homebrew-versions