Sync rna-transcription docs with problem-specifications (#618)

The rna-transcription exercise has been overhauled as part of a project
to make practice exercises more consistent and friendly.

For more context, please see the discussion in the forum, as well as
the pull request that updated the exercise in the problem-specifications
repository:

- https://forum.exercism.org/t/new-project-making-practice-exercises-more-consistent-and-human-across-exercism/3943
- exercism/problem-specifications#2251

Author: kytrinyx

exercises/practice/rna-transcription/.docs/instructions.md

@@ -1,6 +1,6 @@
 # Instructions
 
-Given a DNA strand, return its RNA complement (per RNA transcription).
+Your task is determine the RNA complement of a given DNA sequence.
 
 Both DNA and RNA strands are a sequence of nucleotides.
 
@@ -14,3 +14,7 @@ Given a DNA strand, its transcribed RNA strand is formed by replacing each nucle
 - `C` -> `G`
 - `T` -> `A`
 - `A` -> `U`
+
+~~~~exercism/note
+If you want to look at how the inputs and outputs are structured, take a look at the examples in the test suite.
+~~~~

exercises/practice/rna-transcription/.docs/introduction.md

@@ -0,0 +1,16 @@
+# Introduction
+
+You work for a bioengineering company that specializes in developing therapeutic solutions.
+
+Your team has just been given a new project to develop a targeted therapy for a rare type of cancer.
+
+~~~~exercism/note
+It's all very complicated, but the basic idea is that sometimes people's bodies produce too much of a given protein.
+That can cause all sorts of havoc.
+
+But if you can create a very specific molecule (called a micro-RNA), it can prevent the protein from being produced.
+
+This technique is called [RNA Interference][rnai].
+
+[rnai]: https://admin.acceleratingscience.com/ask-a-scientist/what-is-rnai/
+~~~~