QueuePool: support subsecond timeout

QueuePool: support subsecond timeout

Fixes: #5719
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### Description
Make it explicit in the documentation and in the default value for the 'timeout'
parameter that `timeout` can be a float. Because Python timing is not
very accurate, warn about the precision.

### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #5710
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5710
Pull-request-sha: 5f4eef8b4aba756d32e14ea41f71ef2919c26b84

Change-Id: I462524b1624ca5cc76d083a1d58e5dc89501c1a9