Improve tmToString and relating document (#8250)

Summary:
Add examples for `skipTrailingZeros` and `zeroPaddingYear`. Skip an extra
branch when `skipTrailingZeros` is disabled.

Pull Request resolved: #8250

Reviewed By: amitkdutta

Differential Revision: D52531337

Pulled By: mbasmanova

fbshipit-source-id: b0a866962c9631fbf21b1a8f3634f581a498c7fa

velox/type/Timestamp.cpp

@@ -306,11 +306,7 @@ std::string Timestamp::tmToString(
     }
   } else {
     while (nanos > 0) {
-      if (nanos % 10 == 0) {
-        out += '0';
-      } else {
-        out += '0' + nanos % 10;
-      }
+      out += '0' + nanos % 10;
       nanos /= 10;
     }
   }

velox/type/Timestamp.h

@@ -42,10 +42,14 @@ struct TimestampToStringOptions {
   // Whether to add a leading '+' when year is greater than 9999.
   bool leadingPositiveSign = false;
 
-  // Whether to skip trailing zeros of fractional part.
+  /// Whether to skip trailing zeros of fractional part. E.g. when true,
+  /// '2000-01-01 12:21:56.129000' becomes '2000-01-01 12:21:56.129'.
   bool skipTrailingZeros = false;
 
-  // Whether padding zeros are added when the digits of year is less than 4.
+  /// Whether padding zeros are added when the digits of year is less than 4.
+  /// E.g. when true, '1-01-01 05:17:32.000' becomes '0001-01-01 05:17:32.000',
+  /// '-03-24 13:20:00.000' becomes '0000-03-24 13:20:00.000', and '-1-11-29
+  /// 19:33:20.000' becomes '-0001-11-29 19:33:20.000'.
   bool zeroPaddingYear = false;
 
   // The separator of date and time.