gpu: nova-core: Clarify sysmembar operations

sysmembar is a critical operation that the GSP falcon needs to perform
in the reset sequence. Add some code comments to clarify.

[acourbot@nvdidia.com: move relevant documentation to SysmemFlush type]

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250708-nova-docs-v4-2-9d188772c4c7@nvidia.com


[ Minor grammar fix in the PFB register documentation. - Danilo ]
Signed-off-by: Danilo Krummrich <dakr@kernel.org>