refactor(human-guides): clarify and add other supported options for `git-crypt add-gpg-user` command in the guide

[KemingHe]

Problem

Current use-cases-git-crypt.md mentions:

common-devx/human-guides/use-cases-git-crypt.md

Lines 125 to 131 in b284c5c

	**Add yourself as a trusted user**:
	```shell
	git-crypt add-gpg-user YOUR_GPG_KEY_ID
	```
	Use your long key ID or fingerprint from [Prerequisites](#prerequisites).

which is inaccurate of official git-crypt README:

https://github.com/AGWA/git-crypt/blob/8c7a90ff38fc9daf41e5f6ccb3f105ee82782231/README.md?plain=1#L43-L51

Proposed Solution

Retain clear, effective, and concise tone/examples while capture original README robust setup content: ID, fingerprint, email, etc. plus ref to gpg man page.

Alternatives Considered

Keep use-cases-git-crypt.md as is - rejected, current wording causes user confusing and is documentation drift from source.