doc/index: Improve get_tz documentation around timezone_name

diff --git a/doc/index.md b/doc/index.md
index 89cea6991837744849d14261cad3216b860adba1..d12f09be981686d09fc07f319d6726d0e50ca8f6 100644 (file)
--- a/doc/index.md
+++ b/doc/index.md
@@ -14,15 +14,17 @@ Returns the current unix timestamp using the most accurate source available.
 See `gettime` for more information.
 
 
-## `get_tz ( timezone_name )`
+## `get_tz ( [timezone_name] )`
 
 Returns a timezone object (see `tzinfo` documentation) for the given `timezone_name`.
+If `timezone_name` is `nil` then the local timezone is used.
+
 This uses the local [zoneinfo database](https://www.iana.org/time-zones); 
-names are usually of the form `Country/Largest_City`. 
+names are usually of the form `Country/Largest_City` e.g. "America/New_York".
 Check [wikipedia](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) for an example list.
 
 
 ## `time_in ( timezone_name [, utc_ts] )`
 
-Returns the current time in seconds since 1970-01-01 0:00:00 in the given timezone
-(as a string, e.g. "America/New_York") at the given UTC time (defaults to now).
+Returns the current time in seconds since 1970-01-01 0:00:00 in the given timezone as a string,
+(same semantics as `get_tz`) at the given UTC time (defaults to now).