src/rados: rewrite bs cache autotune instructions

This commit rewrites one sentence in the "Bluestore
Config Reference". The sentence does not itself appear
in the directory structure under /docs/rados/, but is
instead in /src/common/options/global.yaml.in and is
included in the Bluestore Config Reference by means of
a confval directive.

The rewritten sentence has been rewritten from the passive
voice to the active voice, and a "how"-formation has been
removed. The removal of the "how"-formation, in my opinion,
makes the sentence go down much smoother for the first-time
reader.

Signed-off-by: Zac Dover <[email protected]>

Author: Zac Dover

src/common/options/global.yaml.in

@@ -4695,10 +4695,10 @@ options:
   level: dev
   desc: The number of seconds to wait between rebalances when cache autotune is enabled.
   fmt_desc: |
-    The number of seconds to wait between rebalances when cache autotune
-    is enabled.  This setting changes how quickly the allocation ratios of
-    various caches are recomputed.  Note:  Setting this interval too small
-    can result in high CPU usage and lower performance.
+    The number of seconds to wait between rebalances when cache autotune is
+    enabled.  `bluestore_cache_autotune_interval` sets the speed at which Ceph
+    recomputes the allocation ratios of various caches. Note: Setting this
+    interval too small can result in high CPU usage and lower performance.
   default: 5
   see_also:
   - bluestore_cache_autotune