Correct relative paths for Play 2.2 classpath extensions

[#58432326]
fix docs

[#58432326]
Support Play 2.2 style Java options

Decorate each Java option by pre-pending '-J'.

[#58432326]
Avoid incompatible minimum and maximum heap sizes

Play 2.2 sets -Xms to a large value by default and when this exceeds
our calculated -Xmx the JVM fails to initialise.

Since we do not normally specify -Xms, set this to a low value for
Play 2.2 only.

[#58432326]
fix typo

[#58432326]