[Task]: (DOC) define HOP regular expressions

### What needs to happen?

https://hop.apache.org/manual/next/pipeline/transforms/textfileinput.html

- "Selecting files using Regular Expressions" gives examples of some unusual regex constructs. Please link to a definition of these regexes
```
# maybe .*userdata.*\.txt is meant?
.userdata.\.txt
# what is ENG? and are there other digits but ENG? And how does this relate to Unicode char classes?
[ENG:A-Z][ENG:0-9].*
```
- specify whether regexes are anchored (as if surrounded by ^...$) or partial-match
- define subheadings (anchors) for "Selecting files using Regular Expressions" and "Accepting filenames from a previous transform"

### Issue Priority

Priority: 3

### Issue Component

Component: Documentation

Open the linked Text File Input documentation page and inspect the “Selecting files using Regular Expressions” and “Accepting filenames from a previous transform” sections. Verify the meanings of the shown constructs, determine whether matching is anchored or partial, then add links, the requested subheadings, and clear explanations to the page.

Written by the indexing model from the issue text.