Doc: Updates to CG, Extending, LM and Paper-defaults.ly

Doc: Updates to CG, Extending, LM and Paper-defaults.ly

Better code formatting in some alists.
Previously, the 110-character lines
made it difficult to read; the new
version is also in line with what
the NR says.

Extending: comment TODO remark Telling
the user that he might be confused about
something (while he doesn’t understand
what exactly that is) only adds to confusion;

Use a properly formatted example;
This should actually show how reformatting
Scheme code makes it easier to read; however,
it uses tabs and the result in both HTML
and PDF is messy.

The new version uses normal spaces only
and should be consistent as well as clear.

Other minor wording and punctuation improvements

Capitalize LilyPond correctly

Fix typo in LM
There was an unmatched parenthesis in the text;
changed into a comma.