Make rule 111 consistent to 109

[ePaul]

• Guideline 109 describes a conservative approach to API design – servers are supposed to reject unknown properties in inputs.
• Guideline 111 says that "servers may reject requests whose entities contain undefined fields".

This sentence (and the following one) should be changed to match the approach in rule 109 (and also link to it).

[pablosaraiva]

• Guideline 109 describes a conservative approach to API design – servers are supposed to reject unknown properties in inputs.
• Guideline 111 says that "servers may reject requests whose entities contain undefined fields".

This sentence (and the following one) should be changed to match the approach in rule 109 (and also link to it).

Both links point to guideline 109.

Here is suggested edit to the issue:

Guideline 111

[ktsypkina]

raised a PR with suggested changes, please have a look!
#793