Clarify tls_config_set_*_file() file I/O semantics

Clarify tls_config_set_*_file() file I/O semantics

tls_config_set_*_file(3) do not just set the file paths like
tls_config_set_*_path(3) do, they do load the given file(s) into memory
directly using tls_config_load_file().

This distinction is important because it means a later tls_connect(3)
will not do any file I/O (at least wrt. those files), which is relevant when
for example pleding without "[rwc]path" after loading files into memory and
before doing tls_connect(3).

The manual's current wording made me use the following due to above way of
pledging a program:
	tls_load_file()
	tls_config_set_ca_mem()
	tls_unload_file()

While in fact a single tls_config_set_ca_file() call does the same.

tls_config.c r1.26 (Aug 2016) change the code but forgot to amend the manual
as noted by tb, thanks.

Feedback OK tb