Update documentation for assert.hasAttribute

[garg3133]

After a recent change in the code for assert.hasAttribute() command (nightwatchjs/nightwatch#3651), it can now only check for a single attribute at one time, as opposed to checking for multiple attributes at the same time as written in docs.

So, the description and example for the assert.hasAttribute command needs to be updated.

https://nightwatchjs.org/api/assert/#assert-hasAttribute

[nothuman2718]

There is no assert.hasAttribute() in the specified link https://nightwatchjs.org/api/assert/#assert-hasAttribute

It is present here https://nightwatchjs.org/api/assert/hasAttribute.html#assert.hasAttribute
But documentation looks correct except that of less verbose.