doc: document STARPU_SILENT

doc/starpu.texi

@@ -1740,7 +1740,13 @@ Do not enforce assertions, saves a lot of time spent to compute them otherwise.
 @subsubsection @code{--enable-verbose}
 @table @asis
 @item @emph{Description}:
-Augment the verbosity of the debugging messages.
+Augment the verbosity of the debugging messages. This can be disabled
+at runtime by setting the environment variable @code{STARPU_SILENT} to
+any value.
+
+@smallexample
+% STARPU_SILENT=1 ./vector_scal
+@end smallexample
 @end table
 
 @node --enable-coverage
@@ -2228,11 +2234,21 @@ the coefficient to be applied to it before adding it to the computation part.
 @subsection Miscellaneous and debug
 
 @menu
+* STARPU_SILENT::               Disable verbose mode
 * STARPU_LOGFILENAME::          Select debug file name
 * STARPU_FXT_PREFIX::           FxT trace location
 * STARPU_LIMIT_GPU_MEM::        Restrict memory size on the GPUs
 @end menu
 
+@node STARPU_SILENT
+@subsubsection @code{STARPU_SILENT} -- Disable verbose mode
+@table @asis
+
+@item @emph{Description}:
+This variable allows to disable verbose mode at runtime when StarPU
+has been configured with the option @code{--enable-verbose}.
+@end table
+
 @node STARPU_LOGFILENAME
 @subsubsection @code{STARPU_LOGFILENAME} -- Select debug file name
 @table @asis