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.

doc/configuration.txt

@@ -693,6 +693,8 @@ The following keywords are supported in the "global" section :
    - 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