Help: Document formatting of DEPRECATION target property

Also add a test case to verify it.

Help/prop_tgt/DEPRECATION.rst

@@ -7,3 +7,8 @@ Deprecation message from imported target's developer.
 
 ``DEPRECATION`` is the message regarding a deprecation status to be displayed
 to downstream users of a target.
+
+The message is formatted as follows:
+
+* Lines that do not start in whitespace are wrapped as paragraph text.
+* Lines that start in whitespace are preserved as preformatted text.

Tests/RunCMake/TargetProperties/Deprecation-stderr.txt

@@ -2,7 +2,11 @@
   The library that is being linked to, testLibDeprecation, is marked as being
   deprecated by the owner\.  The message provided by the developer is:
 
-  Deprecated version\.  Please use latest version
+  Deprecated version:
+
+    This is a long line of preformatted text that would otherwise wrap to multiple lines\.
+
+  Please use latest version\.
 
 Call Stack \(most recent call first\):
   CMakeLists\.txt:[0-9]+ \(include\)

Tests/RunCMake/TargetProperties/Deprecation.cmake

@@ -1,5 +1,8 @@
 add_library(testLibDeprecation STATIC empty.cpp)
-set_property(TARGET testLibDeprecation PROPERTY DEPRECATION "Deprecated version. Please use latest version")
+set_property(TARGET testLibDeprecation PROPERTY DEPRECATION
+  "Deprecated version:
+  This is a long line of preformatted text that would otherwise wrap to multiple lines.
+Please use latest version.")
 
 add_executable(testExe1 empty.cpp)
 target_link_libraries(testExe1 testLibDeprecation)