Update min_date/max_date descriptions to support relative dates

Copied text from https://github.com/nextstrain/augur/blob/c264580153afd99ebd3816023980bdc9c040e141/augur/dates.py#L8-L10

In the future, this should point to a document in Augur (see nextstrain/augur#882)

docs/src/reference/configuration.rst

@@ -291,7 +291,12 @@ min_date
 ~~~~~~~~
 
 -  type: string
--  description: Argument to ``augur filter`` to set the minimum collection date for strains to include in the subsampling set. Dates can be numeric floating point values (e.g., ``2019.74``) or ISO 8601-style strings (e.g., ``2019-10-01``).
+-  description: Argument to ``augur filter`` to set the minimum collection date for strains to include in the subsampling set. Dates can be:
+
+   1. an Augur-style numeric date with the year as the integer part (e.g. 2020.42) or
+   2. a date in ISO 8601 date format (i.e. YYYY-MM-DD) (e.g. '2020-06-04') or
+   3. a backwards-looking relative date in ISO 8601 duration format with optional P prefix (e.g. '1W', 'P1W')
+
 -  examples:
 
    -  ``--min-date 2019-10-01``
@@ -303,7 +308,12 @@ max_date
 ~~~~~~~~
 
 -  type: string
--  description: Argument to ``augur filter`` to set the maximum collection date for strains to include in the subsampling set. Dates can be numeric floating point values (e.g., ``2019.74``) or ISO 8601-style strings (e.g., ``2019-10-01``).
+-  description: Argument to ``augur filter`` to set the maximum collection date for strains to include in the subsampling set. Dates can be:
+
+   1. an Augur-style numeric date with the year as the integer part (e.g. 2020.42) or
+   2. a date in ISO 8601 date format (i.e. YYYY-MM-DD) (e.g. '2020-06-04') or
+   3. a backwards-looking relative date in ISO 8601 duration format with optional P prefix (e.g. '1W', 'P1W')
+
 -  examples:
 
    -  ``--max-date 2021-04-01``
@@ -779,7 +789,12 @@ min_date
 ~~~~~~~~
 
 -  type: float or string
--  description: Minimum collection date for strains to include in the analysis used by ``augur filter --min-date``. Dates can be numeric floating point values (e.g., ``2019.74``) or ISO 8601-style strings (e.g., ``2019-10-01``).
+-  description: Minimum collection date for strains to include in the analysis used by ``augur filter --min-date``. Dates can be:
+
+   1. an Augur-style numeric date with the year as the integer part (e.g. 2020.42) or
+   2. a date in ISO 8601 date format (i.e. YYYY-MM-DD) (e.g. '2020-06-04') or
+   3. a backwards-looking relative date in ISO 8601 duration format with optional P prefix (e.g. '1W', 'P1W')
+
 -  default: ``2019.74``
 
 skip_diagnostics
@@ -951,14 +966,24 @@ min_date
 ~~~~~~~~
 
 -  type: float or string
--  description: Earliest date to estimate frequencies for. Dates can be numeric floating point values (e.g., ``2019.74``) or ISO 8601-style strings (e.g., ``2019-10-01``).
+-  description: Earliest date to estimate frequencies for. Dates can be:
+
+   1. an Augur-style numeric date with the year as the integer part (e.g. 2020.42) or
+   2. a date in ISO 8601 date format (i.e. YYYY-MM-DD) (e.g. '2020-06-04') or
+   3. a backwards-looking relative date in ISO 8601 duration format with optional P prefix (e.g. '1W', 'P1W')
+
 -  default: without value supplied, defaults to 1 year before present
 
 max_date
 ~~~~~~~~
 
 -  type: float or string
--  description: Earliest date to estimate frequencies for. Dates can be numeric floating point values (e.g., ``2021.5``) or ISO 8601-style strings (e.g., ``2021-07-01``). Specifying ``max_date`` overrides ``recent_days_to_censor``.
+-  description: Earliest date to estimate frequencies for. Specifying ``max_date`` overrides ``recent_days_to_censor``. Dates can be:
+
+   1. an Augur-style numeric date with the year as the integer part (e.g. 2020.42) or
+   2. a date in ISO 8601 date format (i.e. YYYY-MM-DD) (e.g. '2020-06-04') or
+   3. a backwards-looking relative date in ISO 8601 duration format with optional P prefix (e.g. '1W', 'P1W')
+
 -  default: without value supplied, defaults to today’s date minus ``recent_days_to_censor`` parameter
 
 recent_days_to_censor