git-p4: test cloning with two dirs, clarify doc

git-p4: test cloning with two dirs, clarify doc

Document how git-p4 currently works when specifying multiple
depot paths:

1.  No branches or directories are named.

2.  Conflicting files are silently ignored---the last change
    wins.

2.  Option --destination is required, else the last path is construed
    to be a directory.

3.  Revision specifiers must be the same on all paths for them to
    take effect.

Test this behavior.

Signed-off-by: Pete Wyckoff <redacted>
Signed-off-by: Junio C Hamano <redacted>