docs/security: show the severity filter status

- Add a sentence above the vulnerability table that lets the user know
  whether or not the table has a severity filter applied.

Example: (The table below has been filtered to show High+ severity)

This is a follow-up to 90850a7 which added generated security pages
for different severity levels. security-m.html (Medium),
security-h.html (High) and security-c.html (Critical).

When I google search for `curl cves` the second result is a link
to security-h.html. It may not be immediately obvious that the table is
filtered and not showing all vulnerabilities.

Closes #305

docs/_security.html

@@ -58,12 +58,16 @@
 <p>
 
 #if SEVERITY == 1
+(The table below has been filtered to show Medium+ severity)
 #include "seclist-m.gen"
 #elif SEVERITY == 2
+(The table below has been filtered to show High+ severity)
 #include "seclist-h.gen"
 #elif SEVERITY == 3
+(The table below has been filtered to show Critical severity)
 #include "seclist-c.gen"
 #else
+(The table below shows vulnerabilities of all severity levels)
 #include "seclist.gen"
 #endif
 SUBTITLE(Retracted security vulnerabilities)