feat: add silent parameter to Messageable.send

[Dorukyum]

Summary

Adds a parameter called silent which applies the MessageFlags.suppress_notifications flag.
Also fixes minor inconsistencies in flag documentation.

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.

[codecov]

Codecov Report

Merging #1913 (34b7194) into master (e18d571) will not change coverage.
The diff coverage is 0.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1913   +/-   ##
=======================================
  Coverage   33.33%   33.33%           
=======================================
  Files          97       97           
  Lines       18753    18753           
=======================================
  Hits         6252     6252           
  Misses      12501    12501           

Flag	Coverage Δ
macos-latest-3.10	33.32% <0.00%> (ø)
macos-latest-3.11	33.32% <0.00%> (ø)
macos-latest-3.8	33.33% <0.00%> (ø)
macos-latest-3.9	33.33% <0.00%> (ø)
ubuntu-latest-3.10	33.32% <0.00%> (ø)
ubuntu-latest-3.11	33.32% <0.00%> (ø)
ubuntu-latest-3.8	33.33% <0.00%> (ø)
ubuntu-latest-3.9	33.33% <0.00%> (ø)
windows-latest-3.10	33.32% <0.00%> (ø)
windows-latest-3.11	33.32% <0.00%> (ø)
windows-latest-3.8	33.33% <0.00%> (ø)
windows-latest-3.9	33.33% <0.00%> (ø)

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

Impacted Files	Coverage Δ
discord/abc.py	22.33% <0.00%> (ø)
discord/flags.py	76.09% <ø> (ø)

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