Fix errors and inconsistencies in Variant format documentation

[iemejia]

Summary

Fix bugs, terminology errors, and inconsistencies in the Variant format specification documents.

Changes

VariantEncoding.md

• Fix BINARY -> BYTE_ARRAY (BINARY is not a Parquet physical type)
• Add note on decimal little-endian vs big-endian difference
• Fix decimal implied-precision formula for val <= 0
• Label undocumented reserved bits in metadata/object/array headers
• Make sorted_strings description consistent across three definitions
• Use INT(N, true) notation consistent with LogicalTypes.md
• Hyphenate compound adjectives ("3 byte" -> "3-byte", etc.)

VariantShredding.md

• Fix Python syntax error: iterating dict yields keys only; add .items() for (name, field) unpacking
• Replace BINARY with BYTE_ARRAY
• Fix comma -> colon inside JSON-like literal in table cell
• Remove trailing space inside backticks in table header
• Use INT(N, true) notation consistent with LogicalTypes.md

Validation

No semantic/behavioral changes to the format specification. All fixes are documentation-only.

Split from #572 for easier review.