Improve '--update' docstring

[jgsogo]

Changelog: Feature: More detailed description for --update argument.
Docs: conan-io/docs#1778

This flag appears in many commands: test, create, install, info, workspace, graph lock

close #7146

[czoido]

I think that part of the confusing behaviour of the --update argument is when it is used with version ranges and revisions at the same time and also that are some commands that are meant to install the new packages in the local cache and other that just want to check if there's a new version. Also there was a bug reported using --update that could also make it less evident.

I think that the docstring for this argument could be something like this for: test, create, install, workspace, graph lock

"Will check the remote and in case a newer version and/or revision of the dependencies exists there, it will install those in the local cache. When using version ranges, it will install the latest version that satisfies the range. Also, when using revisions, it will update to the latest revision for the resolved version range."

and for info:

"Will check if updates of the dependencies exist in the remotes (a new version that satisfies a version range, a new revision or a newer recipe if not using revisions)."

Would it make sense with the behaviour we want for --update @memsharded ?

The issue that originated this PR maybe could be an effect of a bug reported with conan graph lock and --update: #7333