docs: use RFC 2119 keywords (cosmos#9532)

## Description

Recently when discussing NFT standard we stumbled upon the proper use of keywards such as SHOULD, MAY etc ...
Let's add them to our guidelines. 

---

### Author Checklist

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- [x] included the correct `docs:` prefix in the PR title
- [x] targeted the correct branch (see [PR Targeting](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [ ] provided a link to the relevant issue or specification
- [x] followed the [documentation writing guidelines](https://github.com/cosmos/cosmos-sdk/blob/master/docs/DOC_WRITING_GUIDELINES.md)
- [x] reviewed "Files changed" and left comments if necessary
- [ ] confirmed all CI checks have passed

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- [x] confirmed the correct `docs:` prefix in the PR title
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- [ ] tested instructions (if applicable)

Author: robert-zaremba

docs/DOC_WRITING_GUIDELINES.md

@@ -8,6 +8,7 @@
 + Don't abuse `code` format when writing in plain English.
 + Follow Google developer documentation [style guide](https://developers.google.com/style).
 + Check the meaning of words in Microsoft's [A-Z word list and term collections](https://docs.microsoft.com/en-us/style-guide/a-z-word-list-term-collections/term-collections/accessibility-terms) (use the search input!).
++ RFC keywords should be used in technical documents (uppercase) and we recommend to use them in user documentation (lowercase). The RFC keywords are: "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL. They are to be interpreted as described in [RFC 2119](https://datatracker.ietf.org/doc/html/rfc2119).
 
 ## Technical Writing Course
 

docs/architecture/README.md

@@ -36,6 +36,10 @@ If recorded decisions turned out to be lacking, convene a discussion, record the
 
 Read about the [PROCESS](./PROCESS.md).
 
+#### Use RFC 2119 Keywords
+
+When writing ADRs, follow the same best practices for writing RFCs. When writing RFCs, key words are used to signify the requirements in the specification. These words are often capitalized: "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL. They are to be interpreted as described in [RFC 2119](https://datatracker.ietf.org/doc/html/rfc2119).
+
 ## ADR Table of Contents
 
 ### Accepted