add docs on explicit timed API

Author: JoshuaMoelans

docs/platforms/native/common/tracing/instrumentation/custom-instrumentation.mdx

@@ -19,6 +19,21 @@ To capture transactions and spans customized to your organization's needs, you m
 
 <PlatformContent includePath="performance/add-spans-example" />
 
+## Custom Timestamps
+Sometimes systems don't provide a reliable system time. For these cases, the following explicitly timed functions can be used.
+- `sentry_transaction_start_ts`
+- `sentry_transaction_finish_ts`
+- `sentry_transaction_start_child_ts`
+- `sentry_transaction_start_child_ts_n`
+- `sentry_span_start_child_ts`
+- `sentry_span_start_child_ts_n`
+- `sentry_span_finish_ts`
+
+They only differ from their non-timestamped counterparts by taking an additional timestamp parameter, which is the unix epoch offset in microseconds.
+
+When using these functions, you should ensure that the provided timestamps are consistent. It is recommended to use the normal interface with automatic timestamping, and to only use the explicitly timed interface when absolutely necessary.
+
+
 <PlatformContent includePath="performance/concurrency" />
 
 <PlatformContent includePath="performance/connect-errors-spans" />