ARM: add help text for HIGHPTE configuration entry

Add some help text for the HIGHPTE configuration entry.  This comes from
the x86 entry, but reworded to be more a more accurate description of
what this option does.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/Kconfig

@@ -1661,6 +1661,12 @@ config HIGHMEM
 config HIGHPTE
 	bool "Allocate 2nd-level pagetables from highmem"
 	depends on HIGHMEM
+	help
+	  The VM uses one page of physical memory for each page table.
+	  For systems with a lot of processes, this can use a lot of
+	  precious low memory, eventually leading to low memory being
+	  consumed by page tables.  Setting this option will allow
+	  user-space 2nd level page tables to reside in high memory.
 
 config HW_PERF_EVENTS
 	bool "Enable hardware performance counter support for perf events"