improve formatting of argument lists descriptions

[seriousben]

This change improves the readability of the description of arguments.

Before:

noop2("""
noop2 foo bar
"""
arg: String): Boolean

After:

noop2(
  """
  noop2 foo bar
  """
  arg: String
): Boolean

Also, Thank you for gqlgen and for this library. ✋ 5️⃣

[coveralls]

Coverage increased (+0.01%) to 92.155% when pulling 768bccf on seriousben:improve-argument-definition-description into d7ac1ee on vektah:master.

[seriousben]

I think this change is aligned with the reference implementation:

https://github.com/graphql/graphql-js/blob/157edcb4f7dfe9cb2acd3d82831430d983bbfbcf/src/language/__tests__/schema-printer-test.js#L55-L58