Mention function/variable naming possibilities and conventions in the learning module

[angelikatyborska]

There are two pieces of information that are missing from the learning module, that users for a programming language should know:

• what are valid function/variable names
• what are typical function/variable names

We should make sure that the introductions to some of the early concept exercises mention:

• Casing (functions and variables snake_case, module names PascalCase)
• Which characters are allowed in function, variable, and module names (people found it surprising to see an argument named legacy? in one exercise)
• What are some naming conventions
  • boolean functions with a question mark (mention this in the boolean concept?)
  • raising functions with an exclamation mark (mention this in the raising errors concept?)

Review the introductions and find appropriate places to insert this information.