Add descriptions for queries

[maarcingebala]

This PR adds descriptions to queries and query arguments. These changes were made when I was working on the initial version of API reference for our docs (saleor/saleor-docs#45). There is still a lot of descriptions to add but I'm planning on doing it in separate PRs)

Pull Request Checklist

1. Privileged views and APIs are guarded by proper permission checks.
2. All visible strings are translated with proper context.
3. All data-formatting is locale-aware (dates, numbers, and so on).
4. Database queries are optimized and the number of queries is constant.
5. Database migration files are up to date.
6. The changes are tested.
7. GraphQL schema and type definitions are up to date.
8. Changes are mentioned in the changelog.

[django-queries]

Here is the report for e5a0141 (maarcingebala/saleor @ graphql-docs)
Base comparison is 5e4e4a4.

No differences were found. (click me)

# api.benchmark checkout
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  add billing address to checkout     	         38	         38	             20
  add shipping to checkout            	          7	          7	              0
  checkout payment charge             	         14	         14	              0
  complete checkout                   	          6	          6	              0
  create checkout                     	         52	         52	             24

# api.benchmark homepage
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve main menu                  	          5	          5	              0
  retrieve product list               	          4	          4	              0
  retrieve secondary menu             	          5	          5	              0
  retrieve shop                       	          2	          2	              0

# api.benchmark product
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  product details                     	         15	         15	              3

# api.benchmark variant
  test name                           	left count 	right count	duplicate count
  ------------------------------------	-----------	-----------	---------------
  retrieve variant list               	         18	         18	              8

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@83c0435). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #4758   +/-   ##
=========================================
  Coverage          ?   91.21%           
=========================================
  Files             ?      330           
  Lines             ?    19880           
  Branches          ?     1882           
=========================================
  Hits              ?    18134           
  Misses            ?     1227           
  Partials          ?      519

Impacted Files	Coverage Δ
saleor/graphql/giftcard/schema.py	100% <ø> (ø)
saleor/graphql/menu/schema.py	100% <ø> (ø)
saleor/graphql/discount/schema.py	100% <ø> (ø)
saleor/graphql/product/schema.py	100% <ø> (ø)
saleor/graphql/shipping/schema.py	100% <ø> (ø)
saleor/graphql/page/schema.py	100% <ø> (ø)
saleor/graphql/order/schema.py	100% <ø> (ø)
saleor/graphql/extensions/schema.py	100% <ø> (ø)
saleor/graphql/account/mutations/base.py	98.52% <100%> (ø)
saleor/graphql/translations/schema.py	98.21% <100%> (ø)
... and 6 more

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[korycins]

Looks good. Happy to see PR from you!