Document how properties work in defaults modules.

Also remove an unused field in DefaultsModuleBase.

Test: m nothing
Test: Verify no diffs in ninja files:
  <check out clean tree>
  m nothing && shasum out/*.ninja | sort -k2 > before
  <apply patch>
  m nothing && shasum out/*.ninja | sort -k2 > after
  diff before after
Bug: 112158820
Change-Id: Id819cea10f7af1603b4e4e1b14c0b49afcd0fecd