Do not list several terms within a single <term> tag

Docs use a mix of incorrect <term>foo, bar</term> and incorrect <term>foo; bar</term> and correct <term>foo</term><term>bar</term>.

Replace some incorrect ones to reduce ambiguity (one option named "foo, bar" versus two options "foo" and "bar"?) and to allow easier reuse of GIMP translations when translators use a Translation Memory.

Also correct capitalization mistakes and in some cases split several options mangled into a single <varlistentry> for no obvious reason.

Note that this is only a partial patch.