Amend WindowFrame docs

The note about WindowFrameBound::Following being only valid "in
WindowFrame::end_bound" was both

- confusing, as it was based on the ANSI SQL syntax the parser doesn't
  adhere to -- though it sounded like a promise about the AST one could
  expect to get from the parser
- and incomplete, as the reality is that the bounds validation the SQL
  engine might want to perform is more complex. For example Postgres
  documentation says <https://www.postgresql.org/docs/11/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS>:

> Restrictions are that frame_start cannot be UNBOUNDED FOLLOWING,
> frame_end cannot be UNBOUNDED PRECEDING, and the frame_end choice
> cannot appear earlier in the above list of frame_start and frame_end
> options than the frame_start choice does — for example RANGE BETWEEN
> CURRENT ROW AND offset PRECEDING is not allowed. But, for example,
> ROWS BETWEEN 7 PRECEDING AND 8 PRECEDING is allowed, even though it
> would never select any rows.

src/parser.rs

@@ -339,7 +339,7 @@ impl Parser {
         })
     }
 
-    /// "CURRENT ROW" | ( (<positive number> | "UNBOUNDED") ("PRECEDING" | FOLLOWING) )
+    /// Parse `CURRENT ROW` or `{ <positive number> | UNBOUNDED } { PRECEDING | FOLLOWING }`
     pub fn parse_window_frame_bound(&mut self) -> Result<WindowFrameBound, ParserError> {
         if self.parse_keywords(vec!["CURRENT", "ROW"]) {
             Ok(WindowFrameBound::CurrentRow)
@@ -347,8 +347,7 @@ impl Parser {
             let rows = if self.parse_keyword("UNBOUNDED") {
                 None
             } else {
-                let rows = self.parse_literal_uint()?;
-                Some(rows)
+                Some(self.parse_literal_uint()?)
             };
             if self.parse_keyword("PRECEDING") {
                 Ok(WindowFrameBound::Preceding(rows))