Fix highlights nested in directive body

The leading two dots of a RST directive gets matched by
rstExplicitMarkup group first, and then the directive name and directive
body will be matched by the groups contained in rstDirectives cluster in
order.

The rstExDirective group in rstDiretives matches any RST directives
other than footnote, citation and hyperlink target, therefore admonition
and code block will be matched by rstExDirective. This fix has
rstExDirective contain rstExplicitMarkup so as to enable recursive RST
directives match.

The interpreted texts enclosed in quotes are not highlighted within a
RST directive body, because the rstCruft cluster contains a non-existing
rstInterpretedText group.It should be renamed to
rstInterpretedTextOrHyperlinkReference which is defined in a
DefineInlineMarkup function call.

syntax/rst.vim

@@ -17,8 +17,9 @@ syn case ignore
 syn match   rstTransition  /^[=`:.'"~^_*+#-]\{4,}\s*$/
 
 syn cluster rstCruft                contains=rstEmphasis,rstStrongEmphasis,
-      \ rstInterpretedText,rstInlineLiteral,rstSubstitutionReference,
-      \ rstInlineInternalTargets,rstFootnoteReference,rstHyperlinkReference
+      \ rstInterpretedTextOrHyperlinkReference,rstInlineLiteral,
+      \ rstSubstitutionReference, rstInlineInternalTargets,rstFootnoteReference,
+      \ rstHyperlinkReference
 
 syn region  rstLiteralBlock         matchgroup=rstDelimiter
       \ start='\(^\z(\s*\).*\)\@<=::\n\s*\n' skip='^\s*$' end='^\(\z1\s\+\)\@!'
@@ -84,7 +85,7 @@ syn region rstHyperlinkTarget matchgroup=rstDirective
 execute 'syn region rstExDirective contained matchgroup=rstDirective' .
       \ ' start=+' . s:ReferenceName . '::\_s+' .
       \ ' skip=+^$+' .
-      \ ' end=+^\s\@!+ contains=@rstCruft,rstLiteralBlock'
+      \ ' end=+^\s\@!+ contains=@rstCruft,rstLiteralBlock,rstExplicitMarkup'
 
 execute 'syn match rstSubstitutionDefinition contained' .
       \ ' /|.*|\_s\+/ nextgroup=@rstDirectives'