Yes, the function docstring and doc page need update

How about replacing

a number of times, as a float measured in seconds. The
with
a number of times. The time is float seconds unless integer nanoseconds for perf_counter_ns. The

I suggest timeit doc

timeit.default_timer()

    The default timer, which is always time.perf_counter().

be expanded to

timeit.default_timer()

    The default timer, which is always time.perf_counter(), returns float seconds.
    The alternative time.perf_counter_ns returns integer nanoseconds.

How about replacing

a number of times, as a float measured in seconds. The

with

a number of times. The time is float seconds unless integer nanoseconds for perf_counter_ns. The

This sounds very clear to me. But since the module lets use any of monotonic, monotonic_ns, perf_counter, perf_counter_ns, process_time, process_time_ns, thread_time, thread_time_ns, time and time_ns, maybe this could say:

… returns the time it takes to execute the main statement a number of times. The time returned is a float measured in seconds, unless a timer function returning nanoseconds is used, in which case, the time returned is an integer measured in nanoseconds.

A bit wordy. Perhaps an example can be inserted in the main documentation?

So,the update description:

timeit.default_timer()

    The default timer, which is  used time.perf_counter(), returns float seconds.
    However,other timer is permitted to substitute,like perf_counter_ns or custom function.
    Note that the time unit of retrun value  depend on the function what used.

Any good advice?

I think a few examples are necessary to clear all bases. In addition to terryjreedy's idea:

… returns the time it takes to execute the main statement a number of times. The return type is a float measured in seconds, unless a different timer function is used. See the examples section for more information.

And maybe just above

You can specify a different timer function to measure the time in different units.

>>> import time
>>> import timeit
>>> timeit.timeit('True != False')  # uses time.perf_counter by default, hence returns a float measured in seconds
0.020857657000306062
>>> timeit.timeit('True != False', timer=time.perf_counter_ns)  # returns an integer measured in nanoseconds
20996968
>>> timeit.timeit('True != False', timer=lambda: time.perf_counter() * 1000)  # returns a float measured in milliseconds
20.884455997496843

The docstring could probably just say that the return value depends on the timer specified.

Is there any progress on this issue? The linked pull does not seem to have been updated.

I plan on choosing from among the alternatives tomorrow to update.

I'm busy with new job recently,thx your work @terryjreedy

The documentation hasn't been updated yet?

I am not proficient with reST, but would be willing to give this a shot using the suggestions here and in #105060.

I have edited the PR and will merge when CI passes.