Basics- pthread mode description is ambiguous

https://brpc.apache.org/docs/server/basics/#pthread-mode

User code(client-side done, server-side CallMethod) runs in bthreads with 1MB stacksize by default. But some of them cannot run in bthreads:

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_**brpc offers pthread mode to solve the issues. When -usercode_in_pthread is turned on, user code will be run in pthreads.**_ Functions that would block bthreads block pthreads.

Note: With -usercode_in_pthread on, brpc::thread_local_data() does not guarantee to return valid value.

Performance issues when pthread mode is on:
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_**User code still runs in special bthreads actually,**_ which use stacks of pthread workers. These special bthreads are scheduled same with normal bthreads and performance differences are negligible.