v3 Docs: Add section for timestamps

[finishes #173542711]

Co-authored-by: Reid Mitchell <rmitchell@pivotal.io>
Co-authored-by: Merric de Launey <mdelauney@pivotal.io>

Author: reidmit

docs/v3/source/includes/concepts/_timestamps.md.erb

@@ -0,0 +1,10 @@
+## Timestamps
+
+Timestamps generally appear in `created_at` and `updated_at` fields on resources.  Precision beyond
+seconds is not supported, even if the underlying database supports it (e.g. Postgres). As a result,
+filtering on sub-second timestamps is not allowed.
+
+All v3 timestamps have the following format `YYYY-MM-DDThh:mm:ssZ`.
+
+Example timestamp (June 30, 2020 at 11:49:04 PM UTC): `2020-06-30T23:49:04Z`
+

docs/v3/source/includes/resources/audit_events/_list.md.erb

@@ -30,7 +30,7 @@ Retrieve all audit events the user has access to.
 
 Name | Type | Description
 ---- | ---- | ------------
-**created_at** (*experimental*)| _iso8601 timestamp_ | Timestamp to filter by. Only supports relational operators, see [relational operators](#relational-operators-experimental)
+**created_at** (*experimental*)| _[timestamp](#timestamps)_ | Timestamp to filter by. Only supports relational operators, see [relational operators](#relational-operators-experimental)
 **types** | _list of strings_ | Comma-delimited list of event types to filter by
 **target_guids** | _list of strings_ | Comma-delimited list of target guids to filter by
 **space_guids** | _list of strings_ | Comma-delimited list of space guids to filter by

docs/v3/source/index.md

@@ -55,6 +55,7 @@ includes:
   - concepts/procfiles
   - concepts/relationships
   - concepts/status_codes
+  - concepts/timestamps
   - concepts/warnings
   - resources/header
   - resources/admin/header