ext4: Document an edge case for overwrites

ext4: Document an edge case for overwrites

ext4_iomap_overwrite_begin() clears the flag for IOMAP_WRITE before
calling ext4_iomap_begin(). Document this above ext4_map_blocks() call
as it is easy to miss it when focusing on write paths alone.

Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Link: https://patch.msgid.link/fd50ba05440042dff77d555e463a620a79f8d0e9.1747337952.git.ritesh.list@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>