PluginCommunication: Footnotes for Arrays of Path Strings

[pryrt]

Adds a description of the memory requirements for Arrays of Path Strings, and links to that from the wchar_t ** messages.
closes #748

(Also fix from the old TCHAR to wchar_t throughout the PluginCommunication, to match the recent Notepad_plus_msgs.h)

[pryrt]

@alankilborn and @molsonkiko, how does this look? Each of the four wchar_t ** messages links to the footnote, and the footnote explains to use MAX_PATH wide characters, which is MAX_PATH*2 bytes.

[alankilborn]

Looks reasonable to me.

[molsonkiko]

@pryrt Yep, that looks like a reasonable change to me. Thanks!