Templates(select) - improve documentation around initial selections

The docs aren't clear about how to go about setting an initial selection for the different selects. It is the same pattern, and the examples use it (setting a selected prop on an option), but it would be helpful to explain this in the docs.

Also, currently you can still pass selected to the templates because the props tables extend SelectProps, but this is currently nonfunctional to a user since state is managed internally. My thought is we can exclude the prop from the extension, and potentially add an initialSelection prop as a more direct way of setting an initial selection (and an alternative to the option prop method that currently exists).

Jira Issue: PF-34

查找模板文档、其中的 select 示例，以及扩展 SelectProps 的 props 表；先比较不同 select 中对现有选中选项模式的描述方式。完成的标准是：初始选择行为得到清晰记录，并且 API 中对 selected 与可能的 initialSelection prop 的处理得到明确，并保持一致地反映出来。