Commit 9c225f26 ("vfs: atomic f_pos accesses as per POSIX") added
locking on read/write/lseek for regular files so that the file position
is updated atomically.

Commit 10dce8af ("fs: stream_open - opener for stream-like files so
that read and write can run simultaneously without deadlock") removed
that locking if a file is stream-like, so that e.g. read don't deadlock
simultaneous write.

Document that file.f_pos_lock is held on read/write & co for regular
files.

In the documentation we can ignore the fact that f_pos_lock is not taken
if there is only one single thread using the file, because it is just an
optimization and does not affect semantic, by which multiple read /
write cannot run simultaneously.
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>