DOC: Update table docstrings to clarify parameters and defaults - #20217

AI disclosure

This PR was entirely generated by AI (Claude Opus 5) and @taldcroft reviewed the changes.

Description

This pull request is to fix inconsistencies between function signatures and their
numpydoc Parameters sections throughout the astropy.table subpackage.

All 22 modules in astropy/table were audited with an AST-based checker that compares
each function/method signature against its docstring in both directions: parameters
documented but absent from the signature, signature parameters missing from the
docstring, parameter ordering disagreement, documented-vs-actual default values, and
__new__/__init__ signature agreement.

The changes are documentation-only — no runtime behavior is modified.

Incorrect documented default (user-visible)

• BaseColumn.pprint documented show_dtype as "Default is True" when the signature
  default is False.

Undocumented parameters in public API

• Column and MaskedColumn class docstrings omitted copy and copy_indices.
  Note that the copy default is None, not True: it is forwarded to
  np.array(..., copy=...) and therefore follows NumPy 2 semantics, where False
  raises ValueError if a copy cannot be avoided. This was verified empirically
  rather than assumed, and is documented accordingly.
• Table.insert_row omitted index from Parameters (it was only described in the
  prose above the section).
• simple_table omitted masked, despite the docstring's own example calling
  simple_table(3, 6, masked=True, kinds='ifOS').

Parameter ordering mismatches

• unique documented keep before silent; the signature is
  unique(input_table, keys=None, silent=False, keep="first"). Both are commonly
  passed positionally, so the documented order was actively misleading.
• notebook_backends.classic documented table_class before css and
  display_length; the signature has it after.

Undocumented parameters in internal functions

• _join: added keys_left and keys_right, reusing the wording already present on
  the public join.
• _vstack: added metadata_conflicts, matching the public vstack wording.
• table_info: added tbl.
• Table._convert_string_dtype: added encode_decode_func. The description was
  checked against the two call sites, which pass np.strings.decode / np.strings.encode
  as a UTF-8 fallback after the fast ASCII path fails.
• TableFormatter._pformat_col, _pformat_col_iter, _pformat_table, and
  _more_tabcol: added the leading col / table / tabcol argument, which is the
  primary subject of each of these methods.
• TableFormatter._pformat_table: fixed "Default is to False" to "Default is False".

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