docs: clarify options for secret types (file, env)

[dvdksn]

Improve descriptions of the attributes for the --secret flag for buildx build.

• Clarify how buildx determines whether to use type=file or type=env.
• Clarify that relative filepaths for type=file are calculated based on CWD where the buildx command is executed.
• Add src and source as possible attributes for type=env, but clarify that they require type=env to be set explicitly if you want to source from an env var (otherwise type=file is assumed).

Related issues

• Closes secret handling documentation for env missing attribute keys #2386

[dvdksn]

check failures are unrelated, but would be nice to make sure validate-docs can run;

ERROR: failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 502 Bad Gateway

I lack permissions to restart the failed jobs.

[crazy-max]

Looks good thanks!