Refactor: based: COMPAT note for cib_op_attr_modifies in primary request

PCMK__CIB_REQUEST_PRIMARY does not modify the CIB. It only puts the
local CIB manager into primary (read/write) mode.

The "modifies CIB" boolean was set to true via commit dd4f9887 in 2008.
No context was given besides "master now needs the config so that it can
set the correct schema version." However, processing a
PCMK__CIB_REQUEST_PRIMARY does not set the schema version.

There might be side effects in legacy mode if we drop the attribute
completely now.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>