fix vcf.rs module doc: add fetch_vcf to header table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/io/vcf.rs

@@ -1,8 +1,16 @@
 //! VCF file I/O utilities.
 //!
-//! All write operations produce BGZF-compressed output with an accompanying
-//! Tabix index (`.tbi`), enabling direct use with `bcftools`, `tabix`, and IGV
-//! without a separate indexing step.
+//! Write operations produce BGZF-compressed output with an accompanying Tabix
+//! index (`.tbi`), enabling direct use with `bcftools`, `tabix`, and IGV without
+//! a separate indexing step. Read operations support both sequential loading and
+//! random-access region queries via the Tabix index.
+//!
+//! | Function | Purpose |
+//! |----------|---------|
+//! | [`read_vcf`] | Load a full VCF file into memory |
+//! | [`write_vcf`] | Write variants to `.vcf.gz` + `.tbi` atomically |
+//! | [`fetch_vcf`] | Random-access region fetch from a Tabix-indexed `.vcf.gz` |
+//! | [`vcf_header`] | Build a full VCF header with contig lines and INFO definitions |
 //!
 //! **Coordinate note:** [`GenomePosition::position`] is 0-based; VCF POS is
 //! 1-based. The conversion `vcf_pos = position + 1` is applied internally.