Clarify docs about magic comments placement

[splattael]

Magic comments like frozen_string_literal may appear everywhere
within the first comment section while encoding have to be the first
line, or second line after shebang.

Examples

frozen_string_literal must appear in the first comment section

# any comment
# It's OK as long it's not a "token"

# frozen_string_literal: true

p ''.frozen? # => true

encoding must be first or second line

# another comment or empty line
# encoding: big5

p ''.encoding # => #<Encoding:UTF-8> so not #<Encoding:Big5>