mention alternatives to compare/2 for comparison in other dates and times

[roelandvanbatenburg]

Similar to #14147

During our upgrade to Elixir 1.18 we got some compile warnings about the usage of == on DateTime structs. https://hexdocs.pm/elixir/1.18.1/DateTime.html suggested using compare/2 which led to replacing

  if user.password_reset_token_sent_at > oldest_allowed_token_datetime do

with

  if DateTime.compare(user.password_reset_token_sent_at, oldest_allowed_token_datetime) == :gt do

However, a far better solution is:

elixir
if DateTime.after?(user.password_reset_token_sent_at, oldest_allowed_token_datetime) do

To make it easier for developers to think of these functions (`after?/2` and `before?/2`), I suggest mentioning them alongside `compare/2`.

[josevalim]

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