Cleanup: move doc-strings into headers, remove duplicates

In some cases move implementation details into the function body.

source/blender/python/generic/py_capi_utils.h

@@ -15,6 +15,7 @@ extern "C" {
 #include "BLI_sys_types.h"
 #include "BLI_utildefines_variadic.h"
 
+/** Useful to print Python objects while debugging. */
 void PyC_ObSpit(const char *name, PyObject *var);
 /**
  * A version of #PyC_ObSpit that writes into a string (and doesn't take a name argument).
@@ -146,6 +147,9 @@ void PyC_MainModule_Restore(PyObject *main_mod);
 
 bool PyC_IsInterpreterActive(void);
 
+/**
+ * Generic function to avoid depending on RNA.
+ */
 void *PyC_RNA_AsPointer(PyObject *value, const char *type_name);
 
 /* flag / set --- interchange */
@@ -211,6 +215,9 @@ struct PyC_StringEnum {
 int PyC_ParseStringEnum(PyObject *o, void *p);
 const char *PyC_StringEnum_FindIDFromValue(const struct PyC_StringEnumItems *items, int value);
 
+/**
+ * Silly function, we don't use arg. just check its compatible with `__deepcopy__`.
+ */
 int PyC_CheckArgs_DeepCopy(PyObject *args);
 
 /* Integer parsing (with overflow checks), -1 on error. */