drm/xe: Document GT statistics

drm/xe: Document GT statistics

In the context of porting applications to SVM, the Xe GT statistics are
used by application developers to validate expected behavior such as
proper alignment, page fault count and migrations. As those statistics
are made for kernel developers, they assume good understanding of driver
internals, which is not always the case on the application side.
Therefore, this commit documents the usage of GT statistics and clarifies
the meaning of identifiers which correspond to the values exposed via
debugfs. Documentation is close to identifiers declaration to make it
easier to maintain when adding new entries in the future.

v2: Fix page reclaim list (PRL) entries (Matthew Brost)

Assisted-by: GitHub Copilot:claude-sonnet-4.6
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/20260327202749.222794-1-francois.dugast@intel.com
Signed-off-by: Francois Dugast <francois.dugast@intel.com>