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)
(cherry picked from commit ceb2d2e)

Author: TheHillBright

man/journald.conf.xml

@@ -260,7 +260,7 @@
         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