DOC: peers: fix doc "enable" statement on "peers" sections

Checking in code the right keyword is "enabled" and not "enable".

In addition the comment was also completed:

This could appear useless because the "defaults" sections not
yet apply on "peers" sections, but it could be the case in the future.
This statement can currently cancel a previous "disabled" keyword in
the same section.

This patch should be backported in all supported branches (keyword
is present since 1.5)
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 2625007..a7e8cdd 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -3027,8 +3027,9 @@
 
   See also: "server" and section 5 about server options
 
-enable
-  This re-enables a disabled peers section which was previously disabled.
+enabled
+  This re-enables a peers section which was previously disabled via the
+  "disabled" keyword.
 
 log <address> [len <length>] [format <format>] [sample <ranges>:<sample_size>]
     <facility> [<level> [<minlevel>]]