Updated docs with percentage information
Updated docs with percentage information

without this change the percentage value might have been set to over 1.0
with this change we explain what are the valid values and what are the reasons for keeping the value as it is

fixes #397

spring-cloud/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,marcingrzejszczak/spring-cloud-sleuth,spring-cloud/spring-cloud-sleuth