Clarified the documentation on vsm_overflow[ed]

In most circumstances the counters vsm_overflow and vsm_overflowed
should be zero. This patch tells the user what he should probably do
when they are not.

include/tbl/vsc_f_main.h

@@ -625,6 +625,8 @@ VSC_F(vsm_overflow,		uint64_t, 0, 'g', 'B', diag,
 	"Number of bytes which does not fit"
 	" in the shared memory used to communicate"
 	" with tools like varnishstat, varnishlog etc."
+	" If this counter is not zero, consider"
+	" increasing the runtime variable vsm_space."
 )
 
 VSC_F(vsm_overflowed,		uint64_t, 0, 'c', 'B', diag,
@@ -632,4 +634,6 @@ VSC_F(vsm_overflowed,		uint64_t, 0, 'c', 'B', diag,
 	"Total number of bytes which did not fit"
 	" in the shared memory used to communicate"
 	" with tools like varnishstat, varnishlog etc."
+	" If this counter is not zero, consider"
+	" increasing the runtime variable vsm_space."
 )