The forgotten assignment operator "=" and the commonplace ":="

The documentation for PL/pgSQL says, that declaration and assignment to variables is done with :=. But a simple, shorter and more modern ^{(see footnote)} = seems to work as expected:

    CREATE OR REPLACE FUNCTION foo() RETURNS int AS $$
    DECLARE
      i int;
    BEGIN
      i = 0;  
      WHILE NOT i = 25 LOOP
          i = i + 1;
          i = i * i;
      END LOOP;
      RETURN i;
    END;
    $$ LANGUAGE plpgsql;

    > SELECT foo();
    25

Please note, that Pl/pgSQL can distinguish assignment and comparison clearly as shown in the line

      WHILE NOT i = 25 LOOP

So, the questions are:

• Didn't I find some section in the docs which mention and/or explains this?
• Are there any known consequences using = instead of :=?

Edit / Footnote:

Please take the "more modern" part with a wink like in A Brief, Incomplete, and Mostly Wrong History of Programming Languages:

1970 - Niklaus Wirth creates Pascal, a procedural language. Critics immediately denounce Pascal because it uses "x := x + y" syntax instead of the more familiar C-like "x = x + y". This criticism happens in spite of the fact that C has not yet been invented.

1972 - Dennis Ritchie invents a powerful gun that shoots both forward and backward simultaneously. Not satisfied with the number of deaths and permanent maimings from that invention he invents C and Unix.