Help: Explain how target SOURCES are interpreted

Co-Authored-By: Craig Scott <craig.scott@crascit.com>

Help/prop_tgt/SOURCES.rst

@@ -1,6 +1,38 @@
 SOURCES
 -------
 
-Source names specified for a target.
+This specifies the list of paths to source files for the target.
+The following commands all set or add to the ``SOURCES`` target property
+and are the usual way to manipulate it:
 
-List of sources specified for a target.
+* :command:`add_executable`
+* :command:`add_library`
+* :command:`add_custom_target`
+* :command:`target_sources`
+
+Contents of ``SOURCES`` may use
+:manual:`generator expressions <cmake-generator-expressions(7)>`.
+If a path starts with a generator expression, it is expected to
+evaluate to an absolute path. Not doing so is considered undefined behavior.
+
+Paths that are for files generated by the build will be treated
+as relative to the build directory of the target, if the path is not
+already specified as an absolute path.  Note that whether a file is seen as
+generated may be affected by policy :policy:`CMP0118`.
+
+If a path does not start with a generator expression, is not an
+absolute path and is not a generated file, it will be treated as relative to
+the location selected by the first of the following that matches:
+
+* If a file by the specified path exists relative to the target's source
+  directory, use that file.
+* If policy :policy:`CMP0115` is not set to ``NEW``, try appending each
+  known source file extension to the path and check if that exists
+  relative to the target's source directory.
+* Repeat the above two steps, this time relative to the target's binary
+  directory instead.
+
+Note that the above decisions are made at generation time, not build time.
+
+See the :manual:`cmake-buildsystem(7)` manual for more on defining
+buildsystem properties.