checkout doc: clarify command line args for "checkout paths" mode

There are "git checkout [-p][<tree-ish>][--][<paths>...]" in the
SYNOPSIS section, and "git checkout [-p][<tree-ish>][--]<paths>..."
as the header for the section that explains the "check out paths
from index/tree-ish" mode.  It is unclear if we require at least one
path, or it is entirely optional.

Actually, both are wrong.  Without the "-p(atch)" option, you must
have <pathspec> (otherwise, with a commit that is a <tree-ish>, you
would be checking out that commit to build a new history on top of
it).  With it, it is already clear that you are checking out paths,
it is optional.  In other words, you cannot omit both.

The source of the confusion is that -p(atch) is described as if it
is just another "optional" part and its description is lumped
together with the non patch mode, even though the actual end user
experience is vastly different.

Let's split the entry into two, and describe the regular mode and
the patch mode separately.  This allows us to make it clear that the
regular mode MUST be given at least one pathspec, that the patch
mode can be invoked with either '-p' or '--patch' but one of these
must be given, and that the pathspec is entirely optional in the
patch mode.

Also, revamp the explanation of "checkout paths" by removing
extraneous description at the beginning, that says "checking out
paths is not checking out a branch".  Explaining what it is for and
when the user wants to use it upfront is the most direct way to help
the readers.

Noticed-by: Robert P J Day
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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