Help: Clarify command-line encoding of target_link_libraries items

Items that have full paths will be quoted automatically.  Other
items are treated as command-line string fragments and get no
automatic escaping.  Document the behavior for each item kind.

Fixes: #18467

Help/command/target_link_libraries.rst

@@ -43,6 +43,9 @@ Each ``<item>`` may be:
   the library instead of using the full path
   (e.g. ``/usr/lib/libfoo.so`` becomes ``-lfoo``).
 
+  The full path to the target's artifact will be quoted/escaped for
+  the shell automatically.
+
 * **A full path to a library file**: The generated link line will
   normally preserve the full path to the file. The buildsystem will
   have a dependency to re-link ``<target>`` if the library file changes.
@@ -62,9 +65,15 @@ Each ``<item>`` may be:
   imported into generated project files.  This is not supported by other
   generators.
 
+  The full path to the library file will be quoted/escaped for
+  the shell automatically.
+
 * **A plain library name**: The generated link line will ask the linker
   to search for the library (e.g. ``foo`` becomes ``-lfoo`` or ``foo.lib``).
 
+  The library name/flag is treated as a command-line string fragment and
+  will be used with no extra quoting or escaping.
+
 * **A link flag**: Item names starting with ``-``, but not ``-l`` or
   ``-framework``, are treated as linker flags.  Note that such flags will
   be treated like any other library link item for purposes of transitive
@@ -78,6 +87,9 @@ Each ``<item>`` may be:
   flags explicitly. The flags will then be placed at the toolchain-defined
   flag position in the link command.
 
+  The link flag is treated as a command-line string fragment and
+  will be used with no extra quoting or escaping.
+
 * **A generator expression**: A ``$<...>`` :manual:`generator expression
   <cmake-generator-expressions(7)>` may evaluate to any of the above
   items or to a :ref:`;-list <CMake Language Lists>` of them.