make read_file_to_str_until_eof doc more explicit

trinity-1686arequested to merge
trinity-1686a/tor:document-read-until-eof into main

the sz_out param is set to what was read (i.e not including the final NULL the function adds for convenience), but could be understood to be set to what was read+1 (including the NULL terminator)

fix #40852 (closed)

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