vsc: Add a long description to MAIN.shm_* counters

lib/libvsc/VSC_main.vsc

@@ -670,25 +670,37 @@
 	:level:	diag
 	:oneliner:	SHM records
 
+	Number of log records written to the shared memory log.
+
 
 .. varnish_vsc:: shm_writes
 	:level:	diag
 	:oneliner:	SHM writes
 
+	Number of individual writes to the shared memory log. A single
+	write may batch multiple records for bufferred tasks.
+
 
 .. varnish_vsc:: shm_flushes
 	:level:	diag
 	:oneliner:	SHM flushes due to overflow
 
+	Number of writes performed before the end of a bufferred task
+	because adding a record to a batch would exceed vsl_buffer.
 
 .. varnish_vsc:: shm_cont
 	:level:	diag
-	:oneliner:	SHM MTX contention
+	:oneliner:	SHM lock contention
+
+	Number of times a write had to wait for the lock.
 
 
 .. varnish_vsc:: shm_cycles
 	:level:	diag
-	:oneliner:	SHM cycles through buffer
+	:oneliner:	SHM cycles through VSL space
+
+	Number of times a write of log records would reach past the end
+	of the shared memory log, cycling back to the beginning.
 
 
 .. varnish_vsc:: backend_req