MFC r227769, 227770, 227771:

MFC r227769, 227770, 227771:

- Add a note to src/Makefile that explains that 'rm' runs twice because
  thesecond invocation only needs to operate on files with the immutable
  flag set.

- Fix the note in r227769 to be less specific to the immutable flag.

- Replace 'chflags' with 'file flags' in a comment, since 'chflags'is a
  command, not a flag itself.

git-svn-id: svn://svn.freebsd.org/base/stable/8@227881 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f