Improve Zserio Language Overview document

[mikir]

Please improve the Zserio Language Overview according to the following report:

• Function documentation could be more explicit, especially considering that they are allowed to use type parameters
• Implicit length arrays in the language overview the code sample implies that they support compound types (Element), but the implementation of the Java runtime explicitly throws when encountering Compound types
• Extend this document to explicitly state that PackableStructure is packable if at least one its field is packable, see Improve Zserio Encoding Guide for packed arrays #677 (comment)

Created from the issue #677.