[Documentation] Mention forget in help string of remove

[ojdo]

Just a low-key documentation-only feature request: mention the existence of chezmoi forget somewhere in the description message of chezmoi rm -h, or in a separate "See also" paragraph.

Motivation: I today first wanted to remove a file only from the source state. Thinking it might be something chezmoi rm --source-only or chezmoi rm --keep-destination, I had to resort to Google, leading me to #3182, which mentioned chezmoi forget.

[halostatue]

This is implemented in #3363 and should be visible with the next release of chezmoi.