Clarify datalog timestamp format and source (#1772)

Author: PeterJohnson

source/docs/software/telemetry/datalog.rst

@@ -10,7 +10,7 @@ Structure of Data Logs
 
 Similar to NetworkTables, data logs have the concept of entries with string identifiers (keys) with a specified data type.  Unlike NetworkTables, the data type cannot be changed after the entry is created, and entries also have metadata--an arbitrary (but typically JSON) string that can be used to convey additional information about the entry such as the data source or data schema.  Also unlike NetworkTables, data log operation is unidirectional--the ``DataLog`` class can only write data logs (it does not support read-back of written values) and the ``DataLogReader`` class can only read data logs (it does not support changing values in the data log).
 
-Data logs consist of a series of timestamped records.  Control records allow starting, finishing, or changing the metadata of entries, and data records record data value changes.
+Data logs consist of a series of timestamped records.  Control records allow starting, finishing, or changing the metadata of entries, and data records record data value changes.  Timestamps are stored in integer microseconds; when running on the RoboRIO, the FPGA timestamp is used (the same timestamp returned by ``Timer.getFPGATimestamp()``).
 
 .. note: For more information on the details of the data log file format, see the `WPILib Data Log File Format Specification <https://github.com/wpilibsuite/allwpilib/blob/main/wpiutil/doc/datalog.adoc>`__.