docs: document ftp_append behavior and parameters

[w3lld1]

I documented ftp_append() in reference/ftp/functions/ftp-append.xml: it uploads the contents of a local file and appends them to the remote file instead of overwriting it. I also filled in the remote path, local source path, and transfer mode descriptions, and removed the undocumented-function warning.

Fixes #5786 for the ftp_append() page only. I have left the other pages in that tracking issue unchanged.

Validation

I checked the behavior against ext/ftp/php_ftp.c and ext/ftp/ftp.c. On PHP 8.5.11, I also ran a local FTP integration probe covering default and explicit binary transfers, preservation of existing remote contents, unchanged local contents, ASCII transfer, and failure for a missing local source. All ten assertions passed.

Using a local PHP 8.5.11 CLI wrapper, I ran:

• php ../docbook-cs/bin/docbook-cs reference/ftp/functions/ftp-append.xml --no-colors — no violations.
• php ../doc-base/configure.php --with-php=/path/to/local/php --disable-libxml-check --enable-xml-details --redirect-stderr-to-stdout --with-lang=en — the complete English manual XML build and Jing validation passed.
• php ../doc-base/scripts/qa/extensions.xml.php --check — passed, with no resulting source changes.
• php ../doc-base/scripts/qa/section-order.php — no files with issues.
• git diff --check — passed.

I did not run PhD HTML rendering.