Doc & scm/: Music-functions and type-predicates.

Doc & scm/: Music-functions and type-predicates.

- Clean up:
    NR 5.6 "Using music functions"
    EL 2.1 "Music functions"
- In Extending 2.1, move `Void functions' to end of section,
  so `Functions without arguments' comes first.
- Use a consistent indentation format for music functions.
- Make some minor formatting/wording changes.
- Categorize predicates in type-p-name-alist.
- Create type-predicates-doc-string to document
  type-p-name-alist automatically.
- Add notation appendix `Predefined type predicates' to
  include type-predicates-doc-string.
- Show predicate name instead of "unknown" in
  type-check error message (provided by Neil Puttock).
- Make `color?' predicate more specific.