Document `Quantity.to_string()`
Document `Quantity.to_string()`

The documentation now also makes sure to distinguish
`Quantity.to_string()` from `UnitBase.to_string()`.

StuartLittlefair/astropy,StuartLittlefair/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,lpsinger/astropy,mhvk/astropy,larrybradley/astropy,StuartLittlefair/astropy,larrybradley/astropy,pllim/astropy,astropy/astropy,larrybradley/astropy,pllim/astropy,pllim/astropy,pllim/astropy,larrybradley/astropy,larrybradley/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,lpsinger/astropy,mhvk/astropy,lpsinger/astropy,astropy/astropy