Update fsdocs tool

[alex-bogomaz]

Hello,

This should fix #721.

Summary of changes:

• use "fsdocs-tool" instead of deprecated "fsharp.formatting.commandtool"
• generate reference documentation for "FsCheck.Xunit" and "FsCheck.NUnit", not just "FsCheck" itself
• update references in "RunningTests.fsx"
• "template.html" : "fsdocs-collection-name-link" is no longer supported, so use "fsdocs-package-project-url" instead
• "template.html" : add additional style for tooltips. Now "FSharp.Formatting" is generating multi-line tooltips without inserting <br /> tag between lines. As a result, tooltips that describe types with multiple members are hard to read. Had to fix this using CSS.