Explain RAM usage.

libraries/dsp/st/doc/st_histogram_status.txt

@@ -9,6 +9,7 @@ new Jira story):
 - add top level functional description (see 2b, 3)
 - explain why common_paged_ram_r_w was not used (see 2a)
 - remove control via MM (see 3, 4, 5a)
+- explain the RAM usage, how many block RAM / signal input (see 3)
 - verify using DC signal (see 5b)
 - verify that sum(bins) has expected constant value every sync interval
   (see 5c)