textops: fix append_hf function

[smititelu]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

append after last eader occurance

[miconda]

Thanks!

[henningw]

Probably the documentation should also be fixed?

https://kamailio.org/docs/modules/devel/modules/textops.html#textops.f.append_hf

"4.24. append_hf(txt[, hdr])

Appends 'txt' as header after first header field or after last 'hdr' header field. "

[miconda]

Pushed some notes to the docs of basic header operations functions, feel free to add more or adjust.

[henningw]

Thank you Daniel