Research and document allowed characters for field names in node and content layers

[alansemenov]

The documentation for Node API says the following:

Some characters are illegal in a property key. Here’s a list of illegal characters:

_ is system reserved prefix

. is the path separator.

[ and ] are array index indicators.

Find out if it's a full list and update the docs if it's not.

For Content layer allowed characters are not described at all. Is it the same as for the Node layer? Should be documented here.

[rymsha]

Find out if it's a full list and update the docs if it's not.

It would be not trivial, unfortunately. We have some issues with allowed characters enonic/xp#6777
It would be better to implement proper ones.