Document the fact that buffers are movable by default.
Care must be taken to not pass them to functions that yield().
Also clarify other things:
- Passing NULL as "ops" to buflib_alloc_ex() causes
buffers to be movable by default (but not shrinkable).
- If you want shrinkable buffers during compaction,
you have to provide a shrink callback.
- To disable buffer movement, you have to pass NULL
for the move_callback inside the callback structure.
- The concept of default callbacks was removed
long ago, remove the only reference of it.
Change-Id: I3bf0ea6b08b507d80a19f3c2c835aca32b3f7800
66df5f3891