[fix] correct small errors in encrypt-digest, IP, JSON, and map function docs

[boluor]

Summary

Fixes 22 small documentation issues across the encrypt-digest, IP, JSON, and map function references. Each item below is independent.

encrypt-digest-functions

• murmur-hash3-64-v2.md — Description typo singed → signed.
• murmur-hash3-u64-v2.md — Description called the cross-referenced function murmur_hash3_64_v2 "the unsigned version"; it is the signed variant. Corrected.
• xxhash-32.md / xxhash-64.md — removed a Note that recommended using xxhash instead of xxhash (a self-referential copy-paste leftover from the murmur docs — xxhash already IS the recommended function).
• murmur-hash3-64.md — list bullet -Note: was missing the space after the dash; corrected to - Note:.
• crc32.md — the top-level sections were h3 (### Description, ### Syntax, …); promoted to h2 to match sibling docs.

ip-functions

• ipv6-cidr-to-range.md — the /48 max address showed only 4 `ffff` groups; corrected to 5 (2001:db8:1:ffff:ffff:ffff:ffff:ffff).
• ipv4-cidr-to-range.md / ipv6-cidr-to-range.md — several result tables had borders narrower than their data rows; widened/re-aligned.
• ipv6-num-to-string.md — example mentioned ::ffff:111.222.33 with only 3 IPv4 octets; corrected to ::ffff:111.222.33.44.
• ipv6-string-to-num.md — two adjacent bullets described NULL handling with the same effect; merged into one.
• ipv6-string-to-num.md / ipv6-string-to-num-or-default.md / ipv6-string-to-num-or-null.md — frontmatter cross-referenced the inverse function as IPv6NumToString; normalized to the canonical casing IPV6_NUM_TO_STRING.
• 12 ip-function files (cut-ipv6, ipv4-cidr-to-range, ipv4-string-to-num, ipv4-string-to-num-or-default, ipv4-to-ipv6, ipv6-cidr-to-range, ipv6-string-to-num, ipv6-string-to-num-or-default, is-ipv4-compat, is-ipv4-mapped, is-ipv4-string, is-ipv6-string) — removed a redundant lowercase ## <func_name> heading sitting between the frontmatter and the canonical ## Description section.
• is-ipv4-compat.md — a note referenced "RFC 4291 IPv4-Mapped IPv6 Address definition" but this doc is about IPv4-Compatible addresses; corrected.

json-functions

• json-array.md — an older ## Arguments / ## Returns section pair was duplicated by ## Parameters / ## Return Value directly below; removed the older duplicates so the doc matches the sibling-doc style.
• json-array-ignore-null.md — typo ingored → ignored.
• json-extract.md — result-table header showed JSON path '$.[2]' while the query used '$[2]'; header now matches the query.
• json-parse.md — a Usage paragraph used JSONB_PARSE / JSONB_PARSE_ERROR_TO_NULL / JSONB_PARSE_ERROR_TO_VALUE; renamed to the canonical JSON_PARSE series so the paragraph matches the rest of the doc.
• strip-null-value.md — the CREATE TABLE my_test(id, v json) example was missing the column type for id (added INT); the example's result block showed 5 rows but the summary said "1 row in set" (corrected to "5 rows in set").

map-functions

• deduplicate-map.md — heading ## Syntaxntax typo; corrected to ## Syntax.

Test plan

• CI doc build passes
• Spot-check the affected pages render correctly (description sentences, the xxhash docs without the self-reference, crc32 with proper h2 headings, ipv4/ipv6 cidr-to-range tables aligned, the cleaned ip-function frontmatter, the merged ipv6-string-to-num bullets, json-array deduplicated sections, json-extract path match, strip-null-value table type and row count)