Fix documentation about indentation syntax

[odersky]

The fixes omit documenting : in extension blocks. These will be disabled
in a separate commit.

There are two reasons for omitted in : in extensions:

• they are not needed, and there should be one way to do things
• they are confusing since everywhere else a : at eol starts a
  new scope, but here it doesn't.

[mbloms]

• they are confusing since everywhere else a : at eol starts a
  new scope, but here it doesn't.

Could you explain what you mean by this, @odersky? Why is this not a new scope?

scala> object List:
     |   given [T: Ordering] as Ordering[List[T]]:
     |     extension (xs: List[T]):
     |      // xs in scope here
     |       def < (ys: List[T]): Boolean = ???
     |       def min(ys: List[T]): List[T] =
     |         // xs and < in scope here
     |          if xs < ys then xs else ys
     |     // xs not in scope, compile error:
     |     def max(ys: List[T]): List[T] = if xs < ys then ys else xs
     | 
10 |    def max(ys: List[T]): List[T] = if xs < ys then ys else xs
   |                                       ^^
   |                                       Not found: xs
10 |    def max(ys: List[T]): List[T] = if xs < ys then ys else xs
   |                                                            ^^
   |                                                           Not found: xs