runtime(rst): Fix doctest block syntax

A doctest block usually spans multiple lines, e.g.

>>> print('this is a Doctest block')
this is a Doctest block

Remove ``oneline`` argument to syntax region as this requirement is not
met.  Consequently, also remove ``display`` as the prerequisite (the
syntax region is on a single line) is no longer met.

----

Recognise '>>>' inside doctest blocks

Recognise subsequent '>>>' prompts in doctest blocks, e.g.

>>> print('this is a Doctest block')
this is a Doctest block
>>> print('this is a second Doctest block')
this is a second Doctest block
A doctest block usually spans multiple lines, e.g.

>>> print('this is a Doctest block')
this is a Doctest block

related: #18566

Signed-off-by: Kirk Roemer <91125534+kirk-roemer@users.noreply.github.com>
Signed-off-by: Marshall Ward <marshall.ward@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/rst.vim

@@ -28,8 +28,11 @@ syn region  rstQuotedLiteralBlock   matchgroup=rstDelimiter
       \ start="::\_s*\n\ze\z([!\"#$%&'()*+,-./:;<=>?@[\]^_`{|}~]\)"
       \ end='^\z1\@!' contains=@NoSpell
 
-syn region  rstDoctestBlock         oneline display matchgroup=rstDelimiter
+syn region  rstDoctestBlock         matchgroup=rstDoctestBlockPrompt
       \ start='^>>>\s' end='^$'
+      \ contains=rstDoctestBlockPrompt
+
+syn match   rstDoctestBlockPrompt   contained '^>>>\s'
 
 syn region  rstTable                transparent start='^\n\s*+[-=+]\+' end='^$'
       \ contains=rstTableLines,@rstCruft
@@ -256,6 +259,7 @@ hi def link rstTransition                   rstSections
 hi def link rstLiteralBlock                 String
 hi def link rstQuotedLiteralBlock           String
 hi def link rstDoctestBlock                 PreProc
+hi def link rstDoctestBlockPrompt           rstDelimiter
 hi def link rstTableLines                   rstDelimiter
 hi def link rstSimpleTableLines             rstTableLines
 hi def link rstExplicitMarkup               rstDirective