DOC: Clarify reindexing in column assignment (#39845)

[AKSHAY355-a]

Added subsection documenting Series-to-DataFrame column alignment (index reindexing, NaN-fill).

Changes:

• Bullets: label placement, NaN missing, silent drops
• ipython example: s.index=[1,0] → "bar"@0, "foo"@1, NaN@2
• Note: list/ndarray (positional exact length) vs dict (scalar broadcast)
• After "Setting with enlargement" (~796)

Closes #39845.

Checklist:

• closes DOC: Reindexing behaviour of dataframe column-assignment missing #39845
• Tests... (leave unchecked)
• All code checks passed (CI)
• Type annotations... (skip)
• whatsnew entry... (DOC-only)
• Reviewed contribution guidelines
• AI prompted per AGENTS.md (Antigravity)

[jbrockmendel]

closed by #64274