Mention floating rounding issues

* doc/lispref/numbers.texi (Float Basics): Mention floating-point
rounding issues uncovered by the discussion in Bug#42417.
1 job for emacs-27 in 11 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #6578
test-all

00:00:11

 
Name Stage Failure
failed
test-all Test
error: cannot lock ref 'refs/remotes/origin/master': Unable to create '/builds/emacs/emacs/.git/refs/remotes/origin/master.lock': File exists.

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an editor opened by 'git commit'. Please make sure all processes
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fatal: failed to run reflog
ERROR: Job failed: exit code 1