Help: Format and revise file() command documentation

Organize the documentation by sub-command to keep the signatures and
their descriptions nearby.  Use inline and explicit reST markup.  Revise
wording as necessary for the updated layout.  Clarify behavior of the
file(GENERATE) command w.r.t. conflicting file names.
Merge topic 'doc-file-command' into next

d74ed543 Help: Format and revise file() command documentation