DOC: configuration: add missing index entries for tune.pool-{low,high}-fd-ratio

These two keywords didn't have an entry in the index. They were added in
2.0 by commit 88698d966 ("MEDIUM: connections: Add a way to control the
number of idling connections.") so this must be backported to 2.0.

(cherry picked from commit a8e2d9790548f395c5f3782117ef2eef350c7c39)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 2f20960904eb35b36baf4e2d279f1344743f50e8)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 3b6d56e..16aa228 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -671,6 +671,8 @@
    - tune.maxrewrite
    - tune.pattern.cache-size
    - tune.pipesize
+   - tune.pool-high-fd-ratio
+   - tune.pool-low-fd-ratio
    - tune.rcvbuf.client
    - tune.rcvbuf.server
    - tune.recv_enough