NULL as a (pseudo-)value not described?

As far as I can tell,  
the un-value NULL is not described anywhere in the doc
as being something that can be assigned to a column.

E.g. :
   .  not listed as a constant  (well that's reasonable as it isn't)
    .  not listed as an expression  (is NULL an expression?)
    .  not explicitly listed under the INSERT and UPDATE commands
        as a valid syntactic unit as an alternative to an expression
        (If it is not an expression)

I do see it listed as a valid keyword but there is no description there.

To put it another way,   the following syntax is apparently not permitted :
      UPDATE my_nullable_table SET nullable_col = NULL;

(I am not referring to other null-related constructs such as IS NULL,  IFNULL etc)

I have to assume I'm missing something?
Or is it that NULL is so,  well,  null,   that it cannot be described?

John