Clarify Javadoc for Tagger.withTagContext.
Clarify Javadoc for Tagger.withTagContext.

See census-instrumentation/opencensus-specs#99.

census-instrumentation/opencensus-java,census-instrumentation/opencensus-java,sebright/opencensus-java,sebright/instrumentation-java,bogdandrutu/opencensus-java,songy23/instrumentation-java,bogdandrutu/opencensus-java,songy23/instrumentation-java,bogdandrutu/instrumentation-java,sebright/instrumentation-java,sebright/opencensus-java,bogdandrutu/instrumentation-java,songy23/instrumentation-java,census-instrumentation/opencensus-java,bogdandrutu/opencensus-java,sebright/opencensus-java