revisions.txt: clarify the .. and ... syntax

In <rev1>..<rev2> and <rev1>...<rev2>, if either <rev1> or <rev2> is
omitted, it defaults to 'HEAD'.  Add this detail to the document.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-log.txt: order OPTIONS properly; move <since>..<until>

The OPTIONS section lists <since>..<until> as the first item, but this
is inconsistent with the ordering in SYNOPSIS.  Move it down until it
appears just before [[--] <path>...].

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-log.txt: generalize <since>..<until>

'<since>..<until>' is misleading, as there are many other forms that
'git log' can accept as an argument.  Replace it with <revision range>,
referring to the section "Specifying Ranges" in revisions.txt, and
rewrite the section appropriately.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-log.txt: rewrite note on why "--" may be required

In its current form, the note talks about separating options from
"branch names" and "refnames" in the same sentence.  This is entirely
inaccurate, as <revision range> need not be a set of branch names or
ref names.  Rewrite it to use the word "revision range", to be
consistent with the SYNOPSIS.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
git-shortlog.txt: make SYNOPSIS match log, update OPTIONS

There are broadly two problems with the current SYNOPSIS.  First, it
completely omits the detail that paths can be specified.  Second, it
attempts to list all the options: this is futile as, in addition to
the options unique to it, it accepts all the options that git-rev-list
accepts.  In fixing these problems, make the SYNOPSIS consistent with
that in git-log.txt.  Also add the corresponding sections to OPTIONS.
Save adding the options from rev-list-options.txt for a later patch,
as it requires some work to pick out the options that are relevant to
shortlog.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>