line_consumption: Fix typos

Problem:
  - Some comments contained typos

Solution:
  - Use spell checking to find those typos and correct them
line_consumption: Fix file comment

Problem:
  - The comment describing the file contents is not using the
    doxygen @file notation
  - File comment is below includes but should be above

Solution:
  - Make use of the @file directive
  - Move comment to the top of the file
line_consumption: Remove unnecessary casts

Problems:
  - The type change of one of the parameters in `plines_win_col` makes some
    casts in the function body superfluous
  - The move of the comment about the file contents left some unnecessary
    newlines

Solutions:
  - Remove unnecessary casts in function body of `plines_win_col` to `colnr_T`
  - Remove unnecessary empty lines
line_consumption: Remove unnecessary check for `true`

Problems:
  - `lineFolded` in `src/nvim/fold.c` has a wrong comment claiming
    that it can return the value `MAYBE`, the implementation doesn't
    allow the return of `MAYBE` though
  - An if check in `plines_win_nofill` in `src/nvim/line_consumption`
    checks the return value of `line_folded` for `true` because of
    the mentioned comment.

Solutions:
  - Fix comment of `lineFolded` in `src/nvim/fold.c`
  - Remove unnecessary check for return value of `lineFolded` in
    `plines_win_nofill` of `src/nvim/line_consumption.c`