Changed instances of writeable to writable, added docstrings to _flags.pyx

[ndgrigorian]

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Instances of writeable have been changed to writable throughout dpctl
Closes #924

Docstrings have been added to _flags.pyx. They are currently only user-accessible by calling help(flags) where flags is an instance of dpctl.tensor.Flags.
Closes #927

[coveralls]

Coverage remained the same at 82.136% when pulling 5e40d55 on flags-cleanup into 051e473 on master.

[github-actions]

View rendered docs @ https://intelpython.github.io/dpctl/pulls/928/index.html

[github-actions]

Array API standard conformance tests for dpctl=0.14.0dev0=py310h8c27c75_165 ran successfully.
Passed: 33
Failed: 801
Skipped: 3138

[github-actions]

Array API standard conformance tests for dpctl=0.14.0dev0=py310h8c27c75_165 ran successfully.
Passed: 33
Failed: 801
Skipped: 3138

[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.14.0dev0=py310h8c27c75_165 ran successfully.
Passed: 33
Failed: 801
Skipped: 3138