cmRST: Fix cmake domain directives with newline before argument

The `signature` directive added by commit 74e3c1d313 (Utilities/Sphinx:
Add a directive to document command signatures, 2023-02-24) will be
commonly used with the form:

    .. signature::
       some_command(SOME_SIGNATURE)

      Docs for this signature.

Drop the assumption that all CMake domain directives are immediately
followed by their argument on the same line.

Source/cmRST.cxx

@@ -21,7 +21,7 @@ cmRST::cmRST(std::ostream& os, std::string docroot)
   , DocRoot(std::move(docroot))
   , CMakeDirective("^.. (cmake:)?("
                    "command|envvar|genex|signature|variable"
-                   ")::[ \t]+([^ \t\n]+)$")
+                   ")::")
   , CMakeModuleDirective("^.. cmake-module::[ \t]+([^ \t\n]+)$")
   , ParsedLiteralDirective("^.. parsed-literal::[ \t]*(.*)$")
   , CodeBlockDirective("^.. code-block::[ \t]*(.*)$")

Tests/CMakeLib/testRST.expect

@@ -46,7 +46,8 @@ Bracket Comment Content
 Bracket Comment Content
 ]
 
-.. cmake:command:: some_cmd
+.. cmake:command::
+   some_cmd
 
    Command some_cmd description.
 
@@ -54,7 +55,8 @@ Bracket Comment Content
 
    Command other_cmd description.
 
-.. cmake:envvar:: some_var
+.. cmake:envvar::
+   some_var
 
    Environment variable some_var description.
 
@@ -62,7 +64,8 @@ Bracket Comment Content
 
    Environment variable other_var description.
 
-.. cmake:genex:: SOME_GENEX
+.. cmake:genex::
+   SOME_GENEX
 
    Generator expression SOME_GENEX description.
 
@@ -70,7 +73,8 @@ Bracket Comment Content
 
    Generator expression $<OTHER_GENEX> description.
 
-.. cmake:signature:: some_command(SOME_SIGNATURE)
+.. cmake:signature::
+   some_command(SOME_SIGNATURE)
 
    Command some_command SOME_SIGNATURE description.
 
@@ -78,7 +82,8 @@ Bracket Comment Content
 
    Command other_command OTHER_SIGNATURE description.
 
-.. cmake:variable:: some_var
+.. cmake:variable::
+   some_var
 
    Variable some_var description.
 

Tests/CMakeLib/testRST.rst

@@ -49,7 +49,8 @@ Inline literal ``__`` followed by inline link `Link Text <InternalDest_>`_.
 
 .. cmake-module:: testRSTmod.cmake
 
-.. cmake:command:: some_cmd
+.. cmake:command::
+   some_cmd
 
    Command some_cmd description.
 
@@ -57,7 +58,8 @@ Inline literal ``__`` followed by inline link `Link Text <InternalDest_>`_.
 
    Command other_cmd description.
 
-.. cmake:envvar:: some_var
+.. cmake:envvar::
+   some_var
 
    Environment variable some_var description.
 
@@ -65,7 +67,8 @@ Inline literal ``__`` followed by inline link `Link Text <InternalDest_>`_.
 
    Environment variable other_var description.
 
-.. cmake:genex:: SOME_GENEX
+.. cmake:genex::
+   SOME_GENEX
 
    Generator expression SOME_GENEX description.
 
@@ -73,7 +76,8 @@ Inline literal ``__`` followed by inline link `Link Text <InternalDest_>`_.
 
    Generator expression $<OTHER_GENEX> description.
 
-.. cmake:signature:: some_command(SOME_SIGNATURE)
+.. cmake:signature::
+   some_command(SOME_SIGNATURE)
 
    Command some_command SOME_SIGNATURE description.
 
@@ -81,7 +85,8 @@ Inline literal ``__`` followed by inline link `Link Text <InternalDest_>`_.
 
    Command other_command OTHER_SIGNATURE description.
 
-.. cmake:variable:: some_var
+.. cmake:variable::
+   some_var
 
    Variable some_var description.