Docs/mm/allocation-profiling: describe sysctrl limitations in debug mode

When CONFIG_MEM_ALLOC_PROFILING_DEBUG=y, /proc/sys/vm/mem_profiling is
read-only to avoid debug warnings in a scenario when an allocation is
made while profiling is disabled (allocation does not get an allocation
tag), then profiling gets enabled and allocation gets freed (warning due
to the allocation missing allocation tag).

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: ebdf9ad4ca98 ("memprofiling: documentation")
Reported-by: Andrew Morton <[e-mail]>
Signed-off-by: Suren Baghdasaryan <[e-mail]>
Acked-by: SeongJae Park <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Kent Overstreet <[e-mail]>
Cc: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Ran Xiaokai <[e-mail]>
Cc: SeongJae Park <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/sysctl/vm.rst

···
 
 The default value depends on CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT.
 
+When CONFIG_MEM_ALLOC_PROFILING_DEBUG=y, this control is read-only to avoid
+warnings produced by allocations made while profiling is disabled and freed
+when it's enabled.
+
 
 memory_failure_early_kill
 =========================

Documentation/mm/allocation-profiling.rst

···
 sysctl:
   /proc/sys/vm/mem_profiling
 
+  1: Enable memory profiling.
+
+  0: Disable memory profiling.
+
+  The default value depends on CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT.
+
+  When CONFIG_MEM_ALLOC_PROFILING_DEBUG=y, this control is read-only to avoid
+  warnings produced by allocations made while profiling is disabled and freed
+  when it's enabled.
+
 Runtime info:
   /proc/allocinfo