Clarify snippet placeholder structure

Now grammar implies that placeholder value should be a single `any`
node, while in most applications it can be any number of consecutive
`any` nodes. For example, as described in earlier "Placeholders"
section: `${1:another ${2:placeholder}}`.

Adding `+` quantifier to `any` seems to be more precise.

This seems to also fix the grammar for initial snippet input (which is
implied to be `any`).

Author: echasnovski

_specifications/lsp/3.18/language/completion.md

@@ -934,7 +934,7 @@ ${TM_FILENAME/(.*)\..+$/$1/}
 Below is the grammar for snippets in EBNF ([extended Backus-Naur form, XML variant](https://www.w3.org/TR/xml/#sec-notation)). With `\` (backslash), you can escape `$`, `}` and `\`. Within choice elements, the backslash also escapes comma and pipe characters. Only the characters required to be escaped can be escaped, so `$` should not be escaped within these constructs and neither `$` nor `}` should be escaped inside choice constructs.
 
 ```
-any         ::= tabstop | placeholder | choice | variable | text
+any         ::= (tabstop | placeholder | choice | variable | text)+
 tabstop     ::= '$' int | '${' int '}'
 placeholder ::= '${' int ':' any '}'
 choice      ::= '${' int '|' choicetext (',' choicetext)* '|}'