NSEdoc cleanup

Mostly splitting function summaries (the first paragraph of NSEdoc) from
the body of the description to make the summary indexes shorter and
easier to scan.

Also fixed some unbalanced code tags like <code>foo</table>

nselib/stdnse.lua

@@ -232,6 +232,7 @@ function strsplit(pattern, text)
 end
 
 --- Generate a random string.
+--
 -- You can either provide your own charset or the function will use
 -- a default one which is [A-Z].
 -- @param len Length of the string we want to generate.
@@ -549,7 +550,9 @@ function format_timestamp(t, offset)
 end
 
 --- Format the difference between times <code>t2</code> and <code>t1</code>
--- into a string in one of the forms (signs may vary):
+-- into a string
+--
+-- String is in one of the forms (signs may vary):
 -- * 0s
 -- * -4s
 -- * +2m38s