pstore: Fix kernel-doc warning

Fix the warning for the description of struct persistent_ram_buffer and
improve the descriptions of the other struct members while I'm here.

Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20230818201253.2729485-1-willy@infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>