Improvements and clarifications on Croatian digraphs (not ligatures)

[ivankokan]

#497

[jspitz]

This is a controversial issue, see #497. My personal take is that this something to be addressed in the hyphenation patterns.

[ivankokan]

This PR includes:

1. renaming the Croatian option disableligatures to disabledigraphs without preserving backwards compatibility (it is not needed as the current name is wrong and misleading & such a relatively new option has been barely used) - DŽ, LJ and NJ are simply not ligatures (compared to ambiguous IJ, or truly the ligature &) but digraph letters (correspond to phonemes that differ from consecutive DŽ, LJ and NJ)
2. preserve the current polyglossia's behavior when disableligatures is set as it was initially planned (Croatian digraphs: When the digraph is to be output and the font is missing one, mimic it #497 (comment)) - a common way of typesetting in Croatian & "I do not want digraphs at all when I set this option"
3. change the fallback for missing Unicode digraph in a font: instead of falling back to the common way of typesetting (but orthographically incorrect), in this case the digraph (letter) gets mimicked by the two consecutive glyphs - compared to @jspitz's initial idea (Croatian digraphs: When the digraph is to be output and the font is missing one, mimic it #497 (comment)), \nobreak is semantically the best choice & it is being used in many other places within polyglossia (along with its equivalent \penalty\@M)
4. carefully improved manual when it comes to terms "letters", "glyphs", "characters" etc.

[jspitz]

Please provide a PR with only the renaming, not change in function. Please provide an alias disableligatures > disabledigraphs for backwards compatibility.