Improve cis and cispi docs

[GlenHertz]

I couldn't understand the old docs ("Return \exp(iz)") so I tried to improve them. The iz has no space so I was confused it meant im*z (and as an EE I tend to think j is imaginary). Also the \exp part isn't that great in the REPL as it is a bit confusing what that means. I tried typing \exp in the REPL thinking it may expand to Euler's number but it doesn't work. I guess it is for Latex rendering but that kind of messes up the REPL? I tried to keep with the same format for consistency.

Changes:

1. Merged cis(real) and cis(complex) into one doc (hopefully I did that right).
2. More verbose description (adding the benefits of the function).
3. Explained why it is called cis.
4. Added more cross-references

[GlenHertz]

Thanks for the inputs. I've taken another go at it. Please review.

[mcabbott]

This still seems a bit confusing, why write the same formula twice in a sentence, and what exactly does similar mean, does it mean equal?

My earlier suggestion was to separate two things -- the first line tells you what it returns, that's the basic mathematical fact. The next paragraph tries to explain why this exists instead of exp(im*z), and hence why you may want to use it, which is software detail.

[GlenHertz]

The reason for stating it twice is once in Julia syntax and once in math/latex/textbook syntax so a reader will understand regardless of what background they are coming from. Also Latex doesn't render in the REPL so having the Julia syntax is helpful.

I agree that "Similar" is a bit confusing as it implies it is not the same. I think it is the same up to 0.5 ULP so I changed it to "Same as"

[GlenHertz]

Thanks for the feedback. I tried to address them. Let me know if that is OK.

I fixed the whitespace error with a quick commit and it seemed to have created a race condition. The above whitespace build error doesn't seem to include the latest commit...not sure how to kick it off again.

[GlenHertz]

Is this good to go now?

[GlenHertz]

Took another stab at it. It seems:

1. mcabbot doesn't like Similar to because what is similar (the result or the method, etc.). I also find it confusing.
2. stevengj likes Similar to
3. oscardssmith suggested More efficient method for

So I went with Oscar's suggestion and tried to streamline things a bit (being less repetitive across the doc strings).
It should probably be A more efficient method for but it seems pretty common in technical writing to drop the A.

[ViralBShah]

@GlenHertz Can you rebase this to master?