RFR: 8184225: Fix accessibility and tidy issues in jdk.jshell docs

[Jonathan Gibbons]

Please review some simple changes to the HTML markup for the jdk.jshell 
API docs.
There are no material changes to the specification.

In JdiExecutionControlProvider, an HTML table is updated for accessibility.
In Snippet.Kind, some redundant <P> tags are removed, that were being 
reported by tidy.

Also in Snippet.Kind, it seems strange, but not wrong, to put a sentence 
after the usages
of the @jls tag. That sentence becomes part of the body of the @jls tag, 
and not part
of the main description, which may not be as intended.
However, at this point, I have not changed that part of the text.

JBS: https://bugs.openjdk.java.net/browse/JDK-8184225
Webrev: http://cr.openjdk.java.net/~jjg/8184225/webrev.00

The changes are simple enough that I have not published an updated API.

-- Jon