docs: better explain UseFieldNameByDefault

refs #304

Author: caarlos0

README.md

@@ -305,7 +305,8 @@ $ SECRET=/tmp/secret  \
 If you don't want to set the `env` tag on every field, you can use the
 `UseFieldNameByDefault` option.
 
-It will use the field name as environment variable name.
+It will use the field name to define the environment variable name.
+So, `Foo` becomes `FOO`, `FooBar` becomes `FOO_BAR`, and so on.
 
 Here's an example:
 

env.go

@@ -118,6 +118,8 @@ type Options struct {
 
 	// UseFieldNameByDefault defines whether or not env should use the field
 	// name by default if the `env` key is missing.
+	// Note that the field name will be "converted" to conform with environment
+	// variable names conventions.
 	UseFieldNameByDefault bool
 
 	// Custom parse functions for different types.