user-manual: more explanation of push and pull usage

user-manual: more explanation of push and pull usage

Recently a user on the mailing list complained that they'd read the
manual but couldn't figure out how to keep a couple private repositories
in sync.  They'd tried using push, and were surprised by the effect.

Add a little text in an attempt to make it clear that:
	- Pushing to a branch that is checked out will have odd results.
	- It's OK to synchronize just using pull if that's simpler.

Signed-off-by: "J. Bruce Fields" <redacted>