Clarify calendar add always converts to microseconds

lib/elixir/lib/calendar/datetime.ex

@@ -1622,8 +1622,9 @@ defmodule DateTime do
 
   Accepts an `amount_to_add` in any `unit`. `unit` can be `:day`,
   `:hour`, `:minute`, `:second` or any subsecond precision from
-  `t:System.time_unit/0`. It defaults to `:second`. Negative values
-  will move backwards in time.
+  `t:System.time_unit/0` for convenience but ultimately they are
+  all converted to microseconds. Negative values will move backwards
+  in time and the default precision is `:second`.
 
   This function relies on a contiguous representation of time,
   ignoring timezone changes. For example, if you add one day when there

lib/elixir/lib/calendar/naive_datetime.ex

@@ -402,8 +402,9 @@ defmodule NaiveDateTime do
 
   Accepts an `amount_to_add` in any `unit`. `unit` can be `:day`,
   `:hour`, `:minute`, `:second` or any subsecond precision from
-  `t:System.time_unit/0`. It defaults to `:second`. Negative values
-  will move backwards in time.
+  `t:System.time_unit/0` for convenience but ultimately they are
+  all converted to microseconds. Negative values will move backwards
+  in time and the default precision is `:second`.
 
   ## Examples
 

lib/elixir/lib/calendar/time.ex

@@ -514,8 +514,9 @@ defmodule Time do
 
   Accepts an `amount_to_add` in any `unit`. `unit` can be
   `:hour`, `:minute`, `:second` or any subsecond precision from
-  `t:System.time_unit/0`. It defaults to `:second`. Negative values
-  will move backwards in time.
+  `t:System.time_unit/0` for convenience but ultimately they are
+  all converted to microseconds. Negative values will move backwards
+  in time and the default precision is `:second`.
 
   Note the result value represents the time of day, meaning that it is cyclic,
   for instance, it will never go over 24 hours for the ISO calendar.