CMAKE_DL_LIBS: Clarify which functions the variable covers

The docs only mentioned dlopen() and dlclose(), which reads as if code
calling just dlsym(), dladdr(), etc. doesn't need it.  It does on glibc
before 2.34.

While at it, dropped SunOS from the empty-value examples (it gets "dl")
and listed the current AIX value.

Fixes: #28088

Help/variable/CMAKE_DL_LIBS.rst

@@ -2,24 +2,27 @@ CMAKE_DL_LIBS
 -------------
 
 This variable contains a name of the dynamic loading library, or a list of
-system libraries needed to use the ``dlopen()`` and ``dlclose()`` functions.
+system libraries needed to use the platform's dynamic loading functions.
+On most Unix-like systems, these are the functions declared in the
+``<dlfcn.h>`` header, such as ``dlopen()``, ``dlsym()``, ``dlerror()``, and
+``dlclose()``.
 
 Few examples of the values this variable is set to:
 
 ``dl``
   On most Unix-like systems.
 
-``-lld``
-  On AIX, prior to CMake 4.2.
+``ld``
+  On AIX.  Prior to CMake 4.2, the value was ``-lld``.
 
 ``dld``
   On HP-UX.
 
 ""
-  Empty string value or not set on systems that have ``dl*()`` functions
-  either in the default library that is implicitly linked (e.g., BSD-like
-  systems, Haiku, macOS, SunOS, etc.), or on systems that don't provide these
-  functions (e.g., Windows).
+  Empty string value or not set on systems that provide these functions in
+  the default library that is implicitly linked (e.g., BSD-like systems,
+  Haiku, macOS, etc.), or on systems that don't provide these functions
+  (e.g., Windows).
 
 Examples
 ^^^^^^^^