curl_global_sslset.3: clarify

it is a one time *set*, not necessarily a one time use... it can be
called again if the first call failed or just listed the alternatives.

clarify that the available backends are the ones this build supports

plus add some formatting

Reported-by: Rich Gray
Bug: https://curl.haxx.se/mail/lib-2017-08/0119.html

docs/libcurl/curl_global_sslset.3

@@ -29,25 +29,29 @@ curl_global_sslset - Select SSL backend to use with libcurl
 .BI ", curl_ssl_backend ***" avail ");"
 .ad
 .SH DESCRIPTION
-This function configures at runtime which SSL backend to use with libcurl. This
-function can only be called once, and it must be called \fBbefore\fP
-curl_global_init().
+This function configures at runtime which SSL backend to use with
+libcurl. This function can only be used to select an SSL backend once, and it
+must be called \fBbefore\fP \fIcurl_global_init(3)\fP.
 
-The backend can be identified by the id (e.g. CURLSSLBACKEND_OPENSSL). The
-backend can also be specified via the name parameter (passing -1 as id).
-If both id and name are specified, the name will be ignored. If neither id
-nor name are specified, the function will fail with
-CURLSSLSET_UNKNOWN_BACKEND and set the "avail" pointer to the
-NULL-terminated list of available backends.
+The backend can be identified by the \fIid\fP
+(e.g. \fBCURLSSLBACKEND_OPENSSL\fP). The backend can also be specified via the
+\fIname\fP parameter (passing -1 as \fIid\fP). If both \fIid\fP and \fIname\fP
+are specified, the \fIname\fP will be ignored.
+
+If neither \fIid\fP nor \fPname\fP are specified, the function will fail with
+CURLSSLSET_UNKNOWN_BACKEND and set the \fIavail\fP pointer to the
+NULL-terminated list of available backends. The available backends are those
+that this particular build of libcurl supports.
 
 Upon success, the function returns CURLSSLSET_OK.
 
 If the specified SSL backend is not available, the function returns
-CURLSSLSET_UNKNOWN_BACKEND and sets the "avail" pointer to a NULL-terminated
-list of available SSL backends.
+CURLSSLSET_UNKNOWN_BACKEND and sets the \fIavail\fP pointer to a
+NULL-terminated list of available SSL backends. In this case, you may call the
+function again to try to select a different backend.
 
-The SSL backend can be set only once. If it has already been set, a
-subsequent attempt to change it will result in a CURLSSLSET_TOO_LATE.
+The SSL backend can be set only once. If it has already been set, a subsequent
+attempt to change it will result in a \fBCURLSSLSET_TOO_LATE\fP.
 
 \fBThis function is not thread safe.\fP You must not call it when any other
 thread in the program (i.e. a thread sharing the same memory) is running.
@@ -62,8 +66,8 @@ If this function returns CURLSSLSET_OK, the backend was successfully selected.
 If the chosen backend is unknown (or support for the chosed backend has not
 been compiled into libcurl), the function returns CURLSSLSET_UNKNOWN_BACKEND.
 
-If the backend had been configured previously, or if curl_global_init() has
-already been called, the function returns CURLSSLSET_TOO_LATE.
+If the backend had been configured previously, or if \fIcurl_global_init(3)\fP
+has already been called, the function returns CURLSSLSET_TOO_LATE.
 .SH "SEE ALSO"
 .BR curl_global_init "(3), "
 .BR libcurl "(3) "