Help: improve docs for INSTALL_NAME_DIR

INSTALL_NAME_DIR now mentions BUILD_WITH_INSTALL_NAME_DIR.
The INSTALL_NAME_DIR, INSTALL_RPATH and $<TARGET_RUNTIME_DLLS:tgt>
documentation now mention the other two platforms respectively, to
make it easier for developers who are not familiar with all three platforms,

Help/manual/cmake-generator-expressions.7.rst

@@ -1696,6 +1696,9 @@ In the following, the phrase "the ``tgt`` filename" means the name of the
     section for details.  Many :ref:`Find Modules` produce imported targets
     with the ``UNKNOWN`` type and therefore will be ignored.
 
+On platforms that support runtime paths (``RPATH``), refer to the
+:prop_tgt:`INSTALL_RPATH` target property.
+On Apple platforms, refer to the :prop_tgt:`INSTALL_NAME_DIR` target property.
 
 Export And Install Expressions
 ------------------------------

Help/prop_tgt/INSTALL_NAME_DIR.rst

@@ -6,8 +6,9 @@ Directory name for installed targets on Apple platforms.
 ``INSTALL_NAME_DIR`` is a string specifying the directory portion of the
 "install_name" field of shared libraries on Apple platforms for
 installed targets.  When not set, the default directory used is determined
-by :prop_tgt:`MACOSX_RPATH`.  Policies :policy:`CMP0068` and :policy:`CMP0042`
-are also relevant.
+by :prop_tgt:`MACOSX_RPATH`.  If the :prop_tgt:`BUILD_WITH_INSTALL_NAME_DIR`
+property is set, this will be used already in the build tree.
+Policies :policy:`CMP0068` and :policy:`CMP0042` are also relevant.
 
 This property is initialized by the value of the variable
 :variable:`CMAKE_INSTALL_NAME_DIR` if it is set when a target is
@@ -16,3 +17,7 @@ created.
 This property supports :manual:`generator expressions <cmake-generator-expressions(7)>`.
 In particular, the :genex:`$<INSTALL_PREFIX>` generator expression can be
 used to set the directory relative to the install-time prefix.
+
+On platforms that support runtime paths (``RPATH``), refer to the
+:prop_tgt:`INSTALL_RPATH` target property.
+Under Windows, the :genex:`TARGET_RUNTIME_DLLS` generator expression is related.

Help/prop_tgt/INSTALL_RPATH.rst

@@ -31,3 +31,6 @@ the contents of ``INSTALL_RPATH`` are properly escaped in the
 
 This property supports
 :manual:`generator expressions <cmake-generator-expressions(7)>`.
+
+On Apple platforms, refer to the :prop_tgt:`INSTALL_NAME_DIR` target property.
+Under Windows, the :genex:`TARGET_RUNTIME_DLLS` generator expression is related.