Add comments to Bootstrapper for hinting overridable methods.

[GF-Huang]

BootstrapperBase, StyletIoCBootstrapperBase and Bootstrapper<T>, each has some methods overridable. But everytime I implements my Bootstrapper, I always forgets which one need to call base.XXX(), which one no need. This makes me have to look through the source code.

[canton7]

It's always safe to call the base method, so if in doubt, do that.

[GF-Huang]

Call base at the first or at the last of the override method body?