risc-v/arch.h: revising comments

This revises comments about page tables to help understanding.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

arch/risc-v/include/arch.h

@@ -64,8 +64,9 @@
 /* A task group must have its L1 table in memory always, and the rest can
  * be dynamically committed to memory (and even swapped).
  *
- * In this implementation every level tables besides the final level N are
- * kept in memory always, while the level N tables are dynamically allocated.
+ * In this implementation level tables except the final level N are always
+ * kept in static memory, while the level N tables are always dynamically
+ * allocated. There is one static page per level in `spgtables[]`.
  *
  * The implications ? They depend on the MMU type.
  *