Fix issues discovered during PiT

[pleku]

Mostly documentation clarifications, and some changes to the base starter (based on testing alpha3):

• doc: I would change vaadin-core by vaadin in the 2nd snippet since it's the dependency used in starters (wizard)
• doc: in 3rd snippet a tag is missing
• doc: in 3rd snippet ${flow.version} does not work, it's better remove that line and document that the version is comming from vaadin bom
• doc: 6th snippet has a double comma before 'property ='
• doc: 6th snippet should have imports, at least for Component, to avoid confussion with spring class
• doc: InitParameters.SERVLET_PARAMETER_COMPATIBILITY_MODE does not exist
• project-base: InitParameters.SERVLET_PARAMETER_COMPATIBILITY_MODE comment in FixedVaadinServlet is wrong
• doc: clarify which parts of vaadin cannot be used in OSGi, at least fussion-endpoint module (depends on spring)"

[mshabarov]

clarify which parts of vaadin cannot be used in OSGi, at least fussion-endpoint module (depends on spring) is already mentioned in OSGi limitations section https://github.com/vaadin/docs/blob/master/articles/flow/advanced/osgi-basic.asciidoc#limitations