Fix missing space in error message and docs/docstring issues

[veeceey]

Summary

• Bug fix: Fix missing space in ValueError message in _parse_args() — the error previously read "multidict update sequence element #0has length 3; 2 is required" (note #0has) instead of the intended "multidict update sequence element #0 has length 3; 2 is required"
• Docs fix: Fix CIMultiDictProxy documentation incorrectly stating it is inherited from MultiDict when it actually inherits from MultiDictProxy
• Docs fix: Fix "explicitly hand" → "explicitly by hand" in the istr documentation section
• Typo fix: Fix "anount" → "amount" in two code comments (in update() and merge())
• Docstring fix: Fix mismatched parenthesis/broken RST markup in items() docstrings — was *(key, value) pairs) (unmatched * emphasis and extra )) now ((key, value) pairs)

Details

The error message bug is in the implicit string concatenation on lines 834-835 of _multidict_py.py:

# Before (no space between f-strings):
f"multidict update sequence element #{pos}"
f"has length {len(item)}; 2 is required"
# Produces: "multidict update sequence element #0has length 3; 2 is required"

# After:
f"multidict update sequence element #{pos} "
f"has length {len(item)}; 2 is required"
# Produces: "multidict update sequence element #0 has length 3; 2 is required"

Test plan

• Verified the error message fix produces correct output with a manual test
• All changes are straightforward string/comment fixes with no behavioral impact beyond the corrected error message

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

✅ 245 untouched benchmarks

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_{Comparing veeceey:fix/error-message-and-docs-typos (75a9675) with master (8416839)}

[webknjaz]

Best to send separate changes separately. It's a bad tone to smash together unrelated stuff. You might want to learn about atomic changes and conventions of using Git.
Also, in some places the justification is unclear and needs to be relatable. I don't see any research done into the history/context of asterisks in there, for example.
One change is straightforward, indeed. I wish it was atomic and thus acceptable.
Oh, and the blobs of text don't need to be overly verbose — it's better if they relay exactly enough context and are accurate instead.

[veeceey]

Closing this PR and splitting into separate atomic changes per review feedback:

• Fix missing space in ValueError message #1296 -- Fix missing space in ValueError message
• Fix typo in code comments: "anount" -> "amount" #1297 -- Fix typo in code comments ("anount" -> "amount")
• Fix CIMultiDictProxy inheritance docs and istr typo #1298 -- Fix CIMultiDictProxy inheritance docs and istr typo
• Fix broken RST markup in items() docstrings #1299 -- Fix broken RST markup in items() docstrings