Doc: NR: Move "Modifying alists" from 4.1.2 to 5.3. (issue2767043)

[markpolesky]

Reviewers: ,

Message:
This patch...

1) adds a node to NR 5.3 "Modifying properties" that
   explains alist-modifying in a generic way, and

2) removes the similar material from
   NR 4.1.2 "Page formatting".

However, there are two statements I want to make, but I'm
not certain if they are entirely true:

1) If an alist is a grob property or \paper variable, its
   keys can be modified individually without affecting other
   keys.

Is this true for all grob properties and \paper variables?
Are there any other classes of alists that allow setting
keys individually with nested declarations?

2) Nested declarations will not work for context property
   alists (such as beamExceptions, keySignature,
   timeSignatureSettings, etc.).  These properties can only
   be modified by completely re-defining them as alists.

Is this correct?  Or are there some context property alists
that can accept a nested declaration to set one key?  If the
statement is incorrect, is there a pattern to the
exceptions, or is it some sort of case-by-case basis?

If both statements are correct, is this okay to push?

Thanks!
- Mark

Description:
Doc: NR: Move "Modifying alists" from 4.1.2 to 5.3.

Please review this at http://codereview.appspot.com/2767043/

Affected files:
  M Documentation/notation/changing-defaults.itely
  M Documentation/notation/spacing.itely