Correct and deduplicate docs on "printable" characters

[gnprice]

BPO	37864
Nosy	@vstinner, @ezio-melotti, @gnprice
PRs	bpo-37864: Correct and deduplicate "isprintable" docs; add test. #15300

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-08-15.04:40:26.339>
labels = ['3.8', 'expert-unicode']
title = 'Correct and deduplicate docs on "printable" characters'
updated_at = <Date 2019-08-15.04:43:18.597>
user = 'https://github.com/gnprice'

bugs.python.org fields:

activity = <Date 2019-08-15.04:43:18.597>
actor = 'Greg Price'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Unicode']
creation = <Date 2019-08-15.04:40:26.339>
creator = 'Greg Price'
dependencies = []
files = []
hgrepos = []
issue_num = 37864
keywords = ['patch']
message_count = 1.0
messages = ['349792']
nosy_count = 3.0
nosy_names = ['vstinner', 'ezio.melotti', 'Greg Price']
pr_nums = ['15300']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue37864'
versions = ['Python 3.8']

Linked PRs

• gh-82045: Correct and deduplicate "isprintable" docs; add test. #130118
• [3.12] gh-82045: Correct and deduplicate "isprintable" docs; add test. #130125
• [3.13] gh-82045: Correct and deduplicate "isprintable" docs; add test. #130127

[gnprice]

While working on bpo-36502 and then bpo-18236 about the definition and docs of str.isspace(), I looked closely also at its neighbor str.isprintable().

It turned out that we have the definition of what makes a character "printable" documented in three places, giving two different definitions.

The definition in the comment on _PyUnicode_IsPrintable is inverted, so that's an easy small fix.

With that correction, the two definitions turn out to be equivalent -- but to confirm that, you have to go look up, or happen to know, that those are the only five "Other" categories and only three "Separator" categories in the Unicode character database. That makes it hard for the reader to tell whether they really are the same, or if there's some subtle difference in the intended semantics.

I've taken a crack at writing some improved docs text for a single definition, borrowing ideas from the C comment as well as the existing docs text; and then pointing there from the other places we'd had definitions. PR coming shortly.