Fix Issue 19567

[roscarares]

Fix Issue 19567 - added information about how ftell function works.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19567	enhancement	[std.stdio] Not really helpful documentation of `tell`

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8673"