Updated functions documentation (#4232)

* Updated documentation of arithmetic functions, correct some mistakes, added missing functions

Signed-off-by: Matthew Wells <[email protected]>

* Added a few missing variable definitions

Signed-off-by: Matthew Wells <[email protected]>

* Updated arithmetic operators documentation to include symbol

Signed-off-by: Matthew Wells <[email protected]>

* removed unneeded SELECT and LIMIT

Signed-off-by: Matthew Wells <[email protected]>

* reformated table

Signed-off-by: Matthew Wells <[email protected]>

* updated data types of functions and made some corrections

Signed-off-by: Matthew Wells <[email protected]>

* removed missing brackets, fixed incorrect ifnull documentation

Signed-off-by: Matthew Wells <[email protected]>

* fixed more minor mistakes and confirmed all function examples work

Signed-off-by: Matthew Wells <[email protected]>

* added ticks to all function names to get it to pass the linter

Signed-off-by: Matthew Wells <[email protected]>

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Signed-off-by: Matthew Wells <[email protected]>