tracing: docs: Update histogram doc for .percent/.graph and 'nohitcount'

Update histogram document for .percent/.graph suffixes and 'nohitcount'
option.

Link: https://lore.kernel.org/linux-trace-kernel/166610815604.56030.4124933216911828519.stgit@devnote2Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Tested-by: Tom Zanussi <zanussi@kernel.org>

Documentation/trace/histogram.rst

@@ -25,7 +25,7 @@ Documentation written by Tom Zanussi
 
         hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
           [:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
-          [:clear][:name=histname1][:<handler>.<action>] [if <filter>]
+          [:clear][:name=histname1][:nohitcount][:<handler>.<action>] [if <filter>]
 
   When a matching event is hit, an entry is added to a hash table
   using the key(s) and value(s) named.  Keys and values correspond to
@@ -79,6 +79,8 @@ Documentation written by Tom Zanussi
 	.log2          display log2 value rather than raw number
 	.buckets=size  display grouping of values rather than raw number
 	.usecs         display a common_timestamp in microseconds
+        .percent       display a number of percentage value
+        .graph         display a bar-graph of a value
 	=============  =================================================
 
   Note that in general the semantics of a given field aren't
@@ -137,6 +139,12 @@ Documentation written by Tom Zanussi
   existing trigger, rather than via the '>' operator, which will cause
   the trigger to be removed through truncation.
 
+  The 'nohitcount' (or NOHC) parameter will suppress display of
+  raw hitcount in the histogram. This option requires at least one
+  value field which is not a 'raw hitcount'. For example,
+  'hist:...:vals=hitcount:nohitcount' is rejected, but
+  'hist:...:vals=hitcount.percent:nohitcount' is OK.
+
 - enable_hist/disable_hist
 
   The enable_hist and disable_hist triggers can be used to have one