Document acceptable identifiers

[rfdoell]

The documentation indicates that an entity is anything from the start of a line to a period, e.g., ^.*[.]. However, the following error is raised when an entity contains a space:

Traceback (most recent call last):
...
<traceback ellipsized>
...
grako.exceptions.FailedParse: (2:1) Expecting end of text. :
Example Thing. Some words

for the input graph:

Another. Something
Example Thing. Some words

Another -> Example Thing

Documented constraints for what entities can be would help make debugging graph errors faster.