Present perfectVSPresent perfect continuous

Hello,there! I've got one question. The whole point is that I understand the usage of both tenses, but there are lots of grammar books and their explanations are rather different.

We've got two sentences: I've been teaching English for 2 months and I've taught English for 20 years.

I've found out that the Present Perfect Continuous expresses temporary actions and the present perfect is used for permanent actions that last for a long time. Is it true? Yet, my grammar book (Murphy) says that it's much better to use the Present perfect continuous when we want to emphasize how long the action lasts and there are several examples: it's been raining since lunchtime and she has been doing the same job for 20 years. The second sentence expresses the permanent action,but we still use the present perfect continuous. This is the point that really confuses me. Help me,guys!