Improve documentation around "original_text" vs. "text" variables

[fhightower]

We should clarify the purpose of the original_text` variable.

• make original_text a constant?
• Add docs/comment noting why we parse from original_text to find some ioc types while others are parsed from text

Here's an explanation of the diff between the two variables I shared in another context:

Why are some IOC types are pulled from "original_text" while others are pulled from "text"? The "original_text" points to a string which remains unmodified throughout the code while the "text" variable points to a string which can be modified (e.g. it may have URLs removed from it). When parsing some values, like CVEs, we choose to parse from "original_text" because we want to parse CVEs from URLs even if URLs were removed from the "text" variable. Basically, any IOC data which has a standard (e.g. CVEs or TLDs) is parsed from original_text b/c we want to parse such IOC data from URLs.