4717: Replace incorrect naming rule in LM

4717: Replace incorrect naming rule in LM

This replaces the incorrect rule about naming variables with a
simple reference to the 'alphabetic-only' _convention_.
A thorough explanation in the NR remains to be done.

[simon.albrecht, 2016-01-03 23:17:30]

I promise to learn how to use git cl &c. correctly :-/
For now, it’s no real problem that it’s an entirely new Rietveld issue, since I
didn’t retain anything from the first approach.

Upon your comments, I replaced the former incorrect wording by a statement
merely expressing a _convention_ of using only alphabetic characters.  A
thorough explanation for the naming rules/conventions in the NR needs to be
done, but that seems to be a major task; I couldn’t quite find an appropriate
place.  IMO it /might/ make sense to make an entire new chapter about such
syntax requirements, either between 3.3 and 3.4 or at 3.3.4.

[thomasmorley651, 2016-01-06 18:27:29]

LGTM