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

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/sqlalchemy/engine/create.py b/lib/sqlalchemy/engine/create.py
index 786f8f5d6..b48cead79 100644
--- a/lib/sqlalchemy/engine/create.py
+++ b/lib/sqlalchemy/engine/create.py
@@ -440,9 +440,11 @@ def create_engine(url, **kwargs):
 
             :paramref:`_pool.Pool.reset_on_return`
 
-    :param pool_timeout=30: number of seconds to wait before giving
+    :param pool_timeout=30.0: number of seconds to wait before giving
         up on getting a connection from the pool. This is only used
-        with :class:`~sqlalchemy.pool.QueuePool`.
+        with :class:`~sqlalchemy.pool.QueuePool`. This can be a float but is
+        subject to the limitations of Python time functions which may not be
+        reliable in the tens of milliseconds.
 
     :param pool_use_lifo=False: use LIFO (last-in-first-out) when retrieving
         connections from :class:`.QueuePool` instead of FIFO