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.