new javadoc for Vector3f (#1689)

* Vector3f: correct important javadoc omissions and mistakes

* Vector3f: javadoc frag not a complete sentence, but punctuated like one

* Vector3f: clarify which vectors get modified and which are unaffected

* Vector3f:  a Vector3f has a single "value" with 3 "components"

* Vector3f:  document the nullability of arguments and results

* Vector3f:  no need to emphasize how "easy" chaining is!

* Vector3f:  refer to the components consistently, using capital letters

* Vector3f:  clarify the return values

* Vector3f:  all arithmetic takes place "internally" -- delete the word

* Vector3f:  when there's a single argument, refer to it as "the argument"

* Vector3f:  correct more javadoc mistakes

* Vector3f:  standardize the terminology for caller-provided storage

* Vector3f:  "provided/given" -> "specified" for consistency

* Vector3f:  simplify the javadoc, avoid frequent mention of "this vector"

* Vector3f:  add some details to the javadoc

* Vector3f:  tidy the whitespace

* Vector3f:  contract "it is" to "it's" in javadoc

* Vector3f:  cleanup pass over the javadoc