doc: clarify that struct.pack_into writes 0x00 for pad bytes

[bsder]

BPO	20803
Nosy	@bitdancer, @DonnaDia
PRs	bpo-20803: doc: clarify that struct.pack_into writes 0x00 for pad bytes #28047

^{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 2014-02-28.09:17:29.332>
labels = ['easy', '3.11', 'type-bug', 'docs']
title = 'doc: clarify that struct.pack_into writes 0x00 for pad bytes'
updated_at = <Date 2021-10-09.09:20:20.290>
user = 'https://bugs.python.org/bsder'

bugs.python.org fields:

activity = <Date 2021-10-09.09:20:20.290>
actor = 'komugi'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2014-02-28.09:17:29.332>
creator = 'bsder'
dependencies = []
files = []
hgrepos = []
issue_num = 20803
keywords = ['patch', 'easy']
message_count = 4.0
messages = ['212416', '212433', '400554', '403526']
nosy_count = 5.0
nosy_names = ['bsder', 'r.david.murray', 'docs@python', 'DonnaDia', 'komugi']
pr_nums = ['28047']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20803'
versions = ['Python 3.11']

[bsder]

This code did something unexpected to me:
>>> a = bytearray('1234')
>>> a
bytearray(b'1234')
>>> struct.pack_into('xBxB', a, 0, 0x59, 0x5A)
>>> a
bytearray(b'\x00Y\x00Z')

The unexpected part was that the 'x' pad byte formatter actually *overwrote* the bytes to 0 rather than leaving them alone.

Not necessarily a bug, but the fact that the pad byte writes 0x00 rather than being untouched/ignored should be documented.

[bitdancer]

Contrawise, I would have found it very surprising if it had not padded with null bytes, but yes, the docs could be explicit about it by saying that 'x' padding means inserting null bytes.

[DonnaDia]

I'm working on this

[komugi]

The PR is stale. Can I work on it ?