mm/mmu_gather: update comment on RCU freeing

mm/mmu_gather: update comment on RCU freeing

Some recent discussion on LMKL [0] brought up some interesting and useful
additional context on RCU-freeing for pagetables.

Note down some extra info in here, in particular a) be concrete about the
reason why an arch might not have an IPI and b) add the interesting
paravirt details.

[0] https://lore.kernel.org/linux-kernel/20250206044346.3810242-2-riel@surriel.com/

Link: https://lkml.kernel.org/r/20250211-mmugather-comment-v1-1-1ac1e0c765d2@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>