[fix](doc) string-functions en: backport corner-case examples that exist on zh (6 pages)

[boluor]

Summary

For six string-functions doc pages, the EN side has fewer examples than the ZH side. This PR ports the ZH-only corner-case examples to EN so both pages cover the same set of behaviors.

Page	Examples added to EN	Notes
`hex.md`	+4	whitespace bytes, UTF-8 input, negative two's complement, mixed alphanum
`initcap.md`	+6	empty, non-alphanumeric separators, UTF-8 words, names, sentences, multi-space & adjacent punctuation
`lcase.md`	+4	empty, already-lowercase/numeric, non-alphabetic passthrough, UTF-8 case folding
`ltrim.md`	+6	empty inputs, multi-char substring strip, full match, UTF-8 substring strip, numeric, punctuation
`replace.md`	+4	empty-string edge cases, case-sensitivity, no-match, overlap-free repeats
`xpath-string.md`	+4 (and renamed an existing mislabel)	NULL input rename, CDATA, comments stripped, no-match, malformed XML error

Notes

• I deliberately skipped one ZH `ltrim` example that claimed `LTRIM(' \\t\\n hello world')` returns `hello world` — on Apache Doris 4.1.1 LTRIM only strips ASCII spaces, so the actual output starts with the literal tab. The example was passing the ZH verifier only because its result block contained a real newline that broke the ASCII-table parser. Not porting an example whose documented output is wrong.
• The existing `xpath-string.md` example 5 was titled "Handling CDATA and comments" but its SQL was `xpath_string(NULL, '/a')` — a pure NULL test, no CDATA. Renamed it to "NULL input" and added the actual CDATA / comments / no-match / malformed-XML examples as 6-9.

Verification

Every added example was run against a single-node Apache Doris 4.1.1 cluster; result blocks are the verbatim mysql client output (including column-width padding).

Test plan

• All 28 added examples produce the documented result on 4.1.1.
• Skipped LTRIM \t\n example documented in PR body.
• xpath-string example 5 rename + 6-9 additions verified.
• No deletions of existing examples on any page.

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