[Doc] update docstring of RDN

[zengyh1900]

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Motivation

Currently, MMEditing supports RDN following https://github.com/yjn870/RDN-pytorch/blob/master/models.py#L21 , whose implementation is slightly different from the official implementation: https://github.com/sanghyun-son/EDSR-PyTorch

Modification

Code has been fixed in #1292 and #1311
In this pull request, we add the official implementation in the docstring.

Who can help? @ them here!

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 83.87% // Head: 83.85% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (60f7456) compared to base (08424a7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1326      +/-   ##
==========================================
- Coverage   83.87%   83.85%   -0.02%     
==========================================
  Files         223      223              
  Lines       12736    12736              
  Branches     1990     1990              
==========================================
- Hits        10682    10680       -2     
- Misses       1715     1716       +1     
- Partials      339      340       +1     

Flag	Coverage Δ
unittests	83.85% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmedit/models/backbones/sr_backbones/rdn.py	86.66% <ø> (ø)
...ls/components/stylegan2/generator_discriminator.py	89.75% <0.00%> (-1.21%)	⬇️

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