#239 docs: pass bare substrings to like filters in every example

[RAprogramm]

Closes #239

The generated like binding escapes %, _ and \\ in the value and wraps the result in %...%. Every documented example passed a value that already carried wildcards, so the documented call escaped them into literals and matched nothing.

• Filtering and relations pages in all five wiki languages now pass the bare substring, and the like section states that the macro adds the wildcards and escapes the ones the caller supplies.
• Added the note that like is always a substring match — prefix- or suffix-only matching is not expressible through it.
• The sort_keyset trybuild case stopped modelling the wrong call.
• README attribute reference says the same in one line.
• The difference from search — which binds the term as given, so a % there is a wildcard — is now stated in the README and in all five wiki pages.

The behaviour itself is already covered by the live-Postgres suite: query_filters_and_sorts asserts that a value containing % matches literally.