Elaborate on motivation in template

The template had a very short explanation of the motivation, which
probably led many people into believing that the motivation can be
rather short, compared to the explanations.

But really, the motivation is extremely important and should often
contain a lot of information. This makes those expectations a lot more
explicit. It also explicitly asks for use cases to be listed out, which
can then be referenced in the rest of the RFC to make it clearer how it
solves the problem.

Author: Noratrieb

0000-template.md

@@ -11,7 +11,13 @@ One paragraph explanation of the feature.
 # Motivation
 [motivation]: #motivation
 
-Why are we doing this? What use cases does it support? What is the expected outcome?
+Any changes to Rust should focus on solving a problem that users of Rust are having.
+This section should explain this problem in detail, including necessary background.
+
+It should also contain several specific use cases where this feature can help a user, and explain how it helps.
+This can then be used to guide the design of the feature.
+
+This section is one of the most important sections of any RFC, and can be lengthy.
 
 # Guide-level explanation
 [guide-level-explanation]: #guide-level-explanation