Issues with "<assoc=right>" in doc and parser generator

There are a few issues with the associativity option:

1) In [doc/options.md](https://github.com/antlr/antlr4/blob/dev/doc/options.md#rule-element-options), the associativity option is presented as follows:

> ```antlr
> grammar ExprLR;
>
> expr : expr '^' expr
> | expr '*' expr // match subexpressions joined with '*' operator
> | expr '+' expr // match subexpressions joined with '+' operator
> | INT // matches simple integer atom
> ;
> ```
>

2) The other page mentioning this option is correct but not entirely accurate. In [doc/left-recursion.md](https://github.com/antlr/antlr4/blob/dev/doc/left-recursion.md#formal-rules):

> ```antlr
> e : e '*' e
> | e '+' e
> | e '?' e ':' e
> | e '=' e
> | INT
> ;
> ```
>
> If your 4.0 or 4.1 grammar uses a right-associative ternary operator, you will need to update your grammar to
> include **on the alternative operator**. To smooth the transition, is still allowed on
> token references but it is ignored.
>

If the first rule is right-associative, there is only an alternative operator on its right, so I assume the actual position is before the factor. Unless it's possible to put it inside parentheses or elsewhere, like `e : e '*' e | {pred}? (e '+' e | e '?' e ':' e | INT)`, in which case it's more complicated to explain.

3) If a grammar is compiled using that option with the pre-4.0 format, ANTLR issues the following **warning**:

> warning(157): Calc.g4:10:7: rule expr contains an assoc terminal option in an unrecognized location

It should be an error instead because it's very likely to produce a parser that doesn't correspond to what the user expects, and a simple warning is too easy to overlook. If an element of the parsed grammar is not good, the whole grammar should be rejected with a precise (and, if possible, helpful) error message:

> error(XYZ): Calc.g4:10:7: rule expr contains an assoc terminal option in an unrecognized location. This terminal option should be at the beginning of the factor containing the target operator.

## Note

I could do a pull request with the doc fix, although I'm not entirely sure of the rule, and there seems to be many pending pull requests. But I don't think I'm the right person to modify the warning message—I could try, but only if I'm sure the pull request has a chance to pass, so if that's fine for everyone and if the project is still active.

Start with doc/options.md and doc/left-recursion.md, comparing the associativity examples and the stated placement rules. Reproduce the pre-4.0 grammar warning, then locate the parser-generator code that reports it; done means the documentation is consistent and the invalid option placement receives the intended diagnostic.

Written by the indexing model from the issue text.