Add basic command line tool documentation

This is not as complete as it should eventually be, but is a summary of
showinf/bfconvert/tiffcomment/xmlvalid that is hopefully sufficient for
5.0.0.

See #10329.
merge dev_5_0 -D org -I include -E exclude: PR 921 (Final BF doc updates for 5 release)
merge dev_5_0 -D org -I include -E exclude: PR 927 (Add basic command line tool documentation)