Improve API documents of some IME query APIs

This is a follow up CL to my previous CL [1], which enabled following
IME query APIs to return the result based on the caller's user ID, not
based on the current IME user, even when they get called from
background users.

 * InputMethodManager#getInputMethodList()
 * InputMethodManager#getEnabledInputMethodList()
 * InputMethodManager#getEnabledInputMethodSubtypeList()

This CL just clarifies it in the API documents.

This is purely a document fix.  There should be no behavior change.

 [1]: I192a0f5a1375170d17a4c08af94f23966dbaea8b
      7f8ee4b9

Bug: 13002424
Bug: 122164939
Test: make -j checkbuild
Change-Id: I55f51e211a8a47c01469e2d93b04c5c864f0b9ea