[5.2] Clarify Cache Plugin (#43378)

Adds the highlighted text to the description of the page cache plugin in a perhaps feeble attempt to better explain why it probably is not what the user wants to enable.

administrator/language/en-GB/plg_system_cache.ini

@@ -7,5 +7,5 @@ PLG_CACHE_FIELD_BROWSERCACHE_LABEL="Use Browser Caching"
 PLG_CACHE_FIELD_EXCLUDE_DESC="Specify which URLs you want to exclude from caching, each on a separate line. Regular expressions are supported, eg. <br><strong>about\-[a-z]+</strong> - will exclude all URLs that have 'about-', for example 'about-us', 'about-me', 'about-joomla' etc.<br><strong>/component/users/</strong> - will exclude all URLs that have /component/users/.<br><strong>com_users</strong> - will exclude all Users component pages."
 PLG_CACHE_FIELD_EXCLUDE_LABEL="Exclude URLs"
 PLG_CACHE_FIELD_EXCLUDE_MENU_ITEMS_LABEL="Exclude Menu Items"
-PLG_CACHE_XML_DESCRIPTION="Provides page caching."
+PLG_CACHE_XML_DESCRIPTION="Provides page caching. This is only suitable for sites that have static content or content that does not change frequently."
 PLG_SYSTEM_CACHE="System - Page Cache"

administrator/language/en-GB/plg_system_cache.sys.ini

@@ -3,5 +3,5 @@
 ; License GNU General Public License version 2 or later; see LICENSE.txt
 ; Note : All ini files need to be saved as UTF-8
 
-PLG_CACHE_XML_DESCRIPTION="Provides page caching."
+PLG_CACHE_XML_DESCRIPTION="Provides page caching. This is only suitable for sites that have static content or content that does not change frequently."
 PLG_SYSTEM_CACHE="System - Page Cache"