runtime(yaml): disable multiline_scalar detection by default

There have been many complaints about Yaml indenting too much, because
it considers values to be multi-line by default, which leads to
unintended indenting for (apparently most) users.

So let's hide this feature behind the new feature flag, keep it
simple and prefer single line value key pairs by default.

If you want the old behaviour, set the following value: >

  :let g:yaml_indent_multiline_scalar = 1

If not set, it will indent the same as the previous line.

closes #13845

Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/indent.txt

@@ -1255,5 +1255,11 @@ Example of configuration: >
 This variable is equivalent to `g:vim_indent.line_continuation`.
 It's supported for backward compatibility.
 
+YAML							*ft-yaml-indent*
+
+By default, the yaml indent script does not try to detect multiline scalars. 
+If you want to enable this, set the following variable: >
+
+  let g:yaml_indent_multiline_scalar = 1
 
  vim:tw=78:ts=8:noet:ft=help:norl:

runtime/doc/tags

@@ -7364,6 +7364,7 @@ ft-xhtml-omni	insert.txt	/*ft-xhtml-omni*
 ft-xml-omni	insert.txt	/*ft-xml-omni*
 ft-xml-syntax	syntax.txt	/*ft-xml-syntax*
 ft-xpm-syntax	syntax.txt	/*ft-xpm-syntax*
+ft-yaml-indent	indent.txt	/*ft-yaml-indent*
 ft-yaml-syntax	syntax.txt	/*ft-yaml-syntax*
 ft-zimbu-plugin	filetype.txt	/*ft-zimbu-plugin*
 ft-zsh-syntax	syntax.txt	/*ft-zsh-syntax*