Document %workspace% and other token expansions

The %workspace% token is mentioned in the user manual under the --package_path flag. It would be way easier to find if expansion tokens were documented in a separate section.

It would also be useful to document

• where the expansion tokens are expanded (i.e. which files and flags they are valid in)
• whether there are an other expansion tokens

The description of %workspace% is confusing, since it refers to a "Bazel directory", which seems to mean a directory literally named "bazel". I can't find a precise definition of this term, an it is not mentioned in the concepts page. IIUC a workspace would be a directory containing a "WORKSPACE" file, but that doesn't seem to be what this expansion variable means.