.Net: [ITextSearch] Consider making top a mandatory parameter

[roji]

MEVD made Top a mandatory parameter (#10193), given that there's no reasonable default that would make sense, and it's generally better to have users consider how many results they actually need here. The same logic probably applies to ITextSearch, so it may be a good idea to do the same there.

[markwallace-microsoft]

Also need to align on

• Return values
• Filtering