doc: improve keywords self documentation
Newly introduced keyword follow a different logic compared to previous one. For example, if we take http_user_agent, it applies on content placed before it. This is the contrary for a keyword like tls_sni. But the information provided by list-keywords are the same:
= tls_sni = Description: content modifier to match specifically and only on the TLS SNI buffer Features: No option,payload inspecting keyword = http_user_agent = Description: content modifier to match only on the HTTP User-Agent header Features: No option,payload inspecting keyword
Consequence is that user has not enough information to know how to use it.
At minimum I propose we use
context setter instead of