Improve clarity of signal.siginterrupt docs

[a4eb36e5-6759-47bf-8e24-1aa4d349c6ed]

BPO	25559
Nosy	@vstinner, @bitdancer

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GitHub fields: ```python assignee = None closed_at = None created_at = labels = ['type-feature', 'docs'] title = 'signal.siginterrupt description has typo' updated_at = user = 'https://bugs.python.org/TomMeagher' ``` bugs.python.org fields: ```python activity = actor = 'vstinner' assignee = 'docs@python' closed = False closed_date = None closer = None components = ['Documentation'] creation = creator = 'Tom Meagher' dependencies = [] files = [] hgrepos = [] issue_num = 25559 keywords = [] message_count = 3.0 messages = ['254121', '254132', '254140'] nosy_count = 4.0 nosy_names = ['vstinner', 'r.david.murray', 'docs@python', 'Tom Meagher'] pr_nums = [] priority = 'normal' resolution = None stage = None status = 'open' superseder = None type = 'enhancement' url = 'https://bugs.python.org/issue25559' versions = [] ```

[a4eb36e5-6759-47bf-8e24-1aa4d349c6ed]

"if flag is False, system calls will be restarted when interrupted by signal signalnum, otherwise system calls will be interrupted."

This sentence doesn't make any sense as written. I assume there should be a "not" in there somewhere, but I'm unclear if signal calls are not interrupted when the flag is false, or not interrupted when they are true.

[bitdancer]

The first phase says "restarted when interrupted", the second phrase says "interrupted". So the difference is whether the system call is restarted or left in interrupted state (ie: the signal will propagate, which is confirmed by the second paragraph). I can't see a way to phrase it any more clearly, can you?

[vstinner]

For more information on how Python handles signals, you can also read the PEP-475 (which changed how Python 3.5 handles signals ;-))