Change doc*-Arguments in Annotations to simple Multi-Line-Strings

[mathze]

Is your feature request related to a problem? Please describe.

Kotlin offers multi-line-string (aka Raw-String) support from the beginning but Java does not for a long time.
There for the documentation related Annotation arguments where designed to support multi-line strings by allowing to use Arrays of strings. Time passed and Java now (since Java 13 as feature preview; since java 15 official standard) also supports multi-line-strings (aka. Text Blocks). Therefor there is no need to any longer support string-arrays.

Describe the solution you'd like

@Keywords' docSummary and docDetails arguments should be merge to a doc argument of type String,
@KwdArgs doc argument should be converted to simple type String.

Pro's and Con's
👍 Save arguments
👍 Give more control over documentation to the user.
👍 Code becomes simpler.
👍 Using the string-array approach, forced us to use [] in kotlin. Which would become obsolete.
👎 Breaking change!

Additional context

To mitigate the effects to already existing libraries using the current way we should try to support both ways for a short time.