Missing documentation for GIT_DIFF_OPTS and GIT_EXTERNAL_DIFF

I noticed today, after unsuccessfully trying GIT_DIFF_OPTS=-U2, that
1) it did not work, 2) the doc in 1.4.4.4 tells about using
"GIT_DIFF_OPTS=-c", which does not work either and 3) there is
virtually no more doc about it in HEAD.

It would be useful to document which flags are recognized, and notably that -u<n> has to be used instead of GNU diff's -U<n>.

At the same time, I noticed that GIT_EXTERNAL_DIFF is not described in the diff docs either, although it is described in the main git.txt page (which may not be the place where users would look for it - at least, only my grep aiming to be accurate before composing this mail finally found it).

Best regards,