clarify in a docstring the reasons for not using literal constants within formulae

Author: slayoo

PySDM/physics/__init__.py

@@ -1,7 +1,19 @@
 """
-Physical `PySDM.physics.constants` and simple formulae (essentially one-liners)
+Physical constants and formulae (mostly one-liners)
   that can be automatically either njit-ted or translated to C (in contrast to
-  more complex code that resides in backends)
+  more complex code that resides in backends). Note that any code here should
+  only use constants defined in `PySDM.physics.constants_defaults` and accessible
+  through the first `const` argument of each formula, for the following reasons:
+  - it makes it possible for a user to override value of any constant by
+    passing a `constants` dictionary to the `__init__` of `Formulae`;
+  - it enforces floating-point cast on all constant values making the code behave
+    in the same way on both CPU and GPU backends (yes, please use `const.ONE/const.TWO`
+    instead of `1/2`);
+  - it enables dimensional analysis logic if the code in question is embedded 
+    in `with DimensionalAnalysis:` block - allows checking physical unit consistency
+    within unit tests (disable by default, no runtime overhead);
+  - last but not least, it requires all the constants to be named
+    (thus resulting in more readable, and more reusable code).
 """
 
 from . import (