DOC: Add tune.h2.be.* and tune.h2.fe.* options to table of contents

These new options were introduced in commits
9d7abda787ac76c04a7e68363d7a322918ce28a7 and
ca1027c22fda9d993cbce716163d28b1e4e8aa0f, both of which at in HAProxy 2.8+.
This patch should be backported there.

(cherry picked from commit 6b40cce10a93ab4924d9a3e999c68ca9345e53a6)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 7254582..789a145 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -1172,6 +1172,10 @@
    - tune.disable-fast-forward
    - tune.fail-alloc
    - tune.fd.edge-triggered
+   - tune.h2.be.initial-window-size
+   - tune.h2.be.max-concurrent-streams
+   - tune.h2.fe.initial-window-size
+   - tune.h2.fe.max-concurrent-streams
    - tune.h2.header-table-size
    - tune.h2.initial-window-size
    - tune.h2.max-concurrent-streams