Improve documentation for strongly typed queries about exactonline-api-dotnet-client

[PerplexWouter]

I'll try to whip up a start for the documentation later this week. I think we should probably highlight the following in order:

• Point out that the properties of all models relate to the fields available in the API (but some API's may have restrictions to the field usage, reference API)
• Show an example of selecting the appropriate fields using the Select method, using both multiple params and multiple Select calls
• Examples using Where/And methods
• Since each operator is only applicable to certain data types, it might be worth highlighting the possible combinations (or at the very least, the important ones!)
• Advanced functions (trim, substring, etc). We should probably point out that at this time only string functions are supported (Contains, EndsWith, StartsWith, IndexOf, Substring, ToLower, ToUpper, Trim, Concat). Also I just noticed (while skimming over the OData specifications) that there is a function "Length" which the library currently doesn't support. DateTime, Math, Type and Geo functions are also not implemented (yet) at this time. I reckon we will want to support all Date/Math/Type functions at some point, and forsace the Geo functions? Might be worth making a note on this point as well.
• Example of using SortBy

I think this should cover everything?

[shtrip]

Sounds good.

Advanced functions (trim, substring, etc).

I don't recommend using such functions. For some of them there is a significant performance price so at some point they may not be supported anymore or only very limited.

[PerplexWouter]

I haven't used all aspects of the API extensively yet, but I did run into the issue of having to trim the Account.Code in the where search due to the field being fixed width (which is one of the reasons I added the support for it in the first place). Perhaps this case should be mentioned?

[shtrip]

For account code you can also consider padding it with spaces if you have to use it in a where clause. The Code property in the Account model already has a remark mentioning this issue. Unfortunately the intellisense doesn't show the remark in the editor right now but I think we can fix that the next time we regenerate all the models.