📚 Clarify field documentation

[elbeno]

Problem:

• The docs for field mention the storage in 32-bit words, but this isn't really applicable to a field; it's a message thing (and is mentioned later in that context).
• Field locators may omit dword indices, but this is not documented.
• A couple of important things about field size are not documented.

Solution:

• Remove the reference to storage in the field section.
• Add a note that the max field size is 64 bits.
• Document the alternative located specialization.
• Note the compilation error when storage size exceeds type representation.