journald: clarify doc for usage-related values cap (#37528)

The old description makes users wrongly assume that the cap of 4G
applied, even when the user specifies a value that will result in higher
than 4G. This commit avoids this misunderstanding.

(cherry picked from commit 3308414)
(cherry picked from commit 137d765b2d01dc93a0ec182654ddf4b9b4114bcf)
(cherry picked from commit a7490f121fbecf34d2201d40c238073e1263009c)

Author: TheHillBright

man/journald.conf.xml

@@ -266,7 +266,7 @@
         and use the smaller of the two values.</para>
 
         <para>The first pair defaults to 10% and the second to 15% of
-        the size of the respective file system, but each value is
+        the size of the respective file system, but each of the calculated default values is
         capped to 4G. If the file system is nearly full and either
         <varname>SystemKeepFree=</varname> or
         <varname>RuntimeKeepFree=</varname> are violated when