Added dcumentation for ecm and qs functions

[mayankray2020]

References to other Issues or PRs

Taking over #19971

• ntheory
  • Added documentation of ecm and qs functions

[sympy-bot]

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Hi, I am the SymPy bot (v161). I'm here to help you write a release notes entry. Please read the guide on how to write release notes.

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Here is what the release notes will look like:

• ntheory
  • Added documentation of ecm and qs functions (#20820 by @abh2k and @mayankray2020)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.8.

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#### References to other Issues or PRs
Taking over #19971 
<!-- BEGIN RELEASE NOTES -->
* ntheory
  * Added documentation of ecm and qs functions
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[mayankray2020]

@abh2k @czgdp1807

[abhinav-anand-addepar]

@Yathartha22 Please review this.

[abhinav-anand-addepar]

I have given a minor suggestion. Apart from this it seem good.

[mayankray2020]

@czgdp1807 Can we merge this??

[mayankray2020]

@czgdp1807 can this be merged??