Improve documentation of rez-context --diff

rez-context is super useful to diff rez resolves, but it assumes you're already in one of the rez-created environments to diff.
`rez-context --diff` needs a secondary form which allows you to diff two contexts while not in an existing resolve.
For example: `rez-context --diff /path/to/oldcontext.rxt /path/to/newcontext.rxt`

Start at the rez-context CLI entry point and trace the existing --diff handling, especially its requirement for an active Rez-created environment. Done means `rez-context --diff /path/to/oldcontext.rxt /path/to/newcontext.rxt` can compare two context files without an existing resolve; add or update the relevant CLI tests if present.

Written by the indexing model from the issue text.