Convert thread documentation to mdbook. Also conslidate this man page.

docs/man/libnng.3.adoc

@@ -298,9 +298,9 @@ as a convenience to aid in creating portable applications.
 |xref:nng_opts_parse.3supp.adoc[nng_opts_parse()]|parse command line options
 // |xref:nng_random.3supp.adoc[nng_random()]|get random number
 // |xref:nng_socket_pair.3supp.adoc[nng_socket_pair()]|create connected pair of BSD sockets
-|xref:nng_thread_create.3supp.adoc[nng_thread_create()]|create thread
-|xref:nng_thread_destroy.3supp.adoc[nng_thread_destroy()]|reap thread
-|xref:nng_thread_set_name.3supp.adoc[nng_thread_set_name()]|set thread name
+// |xref:nng_thread_create.3supp.adoc[nng_thread_create()]|create thread
+// |xref:nng_thread_destroy.3supp.adoc[nng_thread_destroy()]|reap thread
+// |xref:nng_thread_set_name.3supp.adoc[nng_thread_set_name()]|set thread name
 |===
 
 === Byte Streams

docs/ref/SUMMARY.md

@@ -6,6 +6,7 @@
 
     - [nng_cv](./api/thr/nng_cv.md)
     - [nng_mtx](./api/thr/nng_mtx.md)
+    - [nng_thread](./api/thr/nng_thread.md)
 
   - [Utility Functions](./api/util/index.md)
 

docs/ref/api/thr/index.md

@@ -6,3 +6,4 @@ are likely to be useful in callback functions and similar situations.
 
 - [nng_cv](nng_cv.md) --- condition variable
 - [nng_mtx](nng_mtx.md) --- mutual exclusion lock
+- [nng_thread](nng_thread.md) -- thread of execution

docs/ref/api/thr/nng_thread.md

@@ -0,0 +1,84 @@
+# nng_thread
+
+## NAME
+
+nng_thread --- thread of execution
+
+## SYNOPSIS
+
+```c
+#include <nng/nng.h>
+
+typedef struct nng_thread nng_thread;
+
+int nng_thread_create(nng_thread **thrp, void (*func)(void *), void *arg);
+void nng_thread_destroy(nng_thread *thr);
+void nng_thread_set_name(nng_thread *thr, const char *name);
+```
+
+### DESCRIPTION
+
+The {{i:`nng_thread`}} structure is used to represent a {{i:thread}} of execution.
+
+In NNG, a thread has an execution fuction _func_, and can be assumed to run this concurrently
+to other threads, including the main thread of the application. The thread persists
+until the function _func_ returns.
+
+> [!TIP]
+> The detail of whether the thread represents an operating system thread,
+> a process, or a "green" thread (also known as a a fiber or coroutine) is determined by the platform.
+> Portable applications should avoid depending on this implementation detail.
+
+The `nng_thread_create` function creates a thread,
+running _func_ with the argument _arg_.
+The thread is started immediately.
+A pointer to the thread object is returned in _thrp_.
+
+Using threads created by this function can make it easy to write
+programs that use simple sequential execution, using functions in the
+_NNG_ suite that would otherwise normally wait synchronously for completion.
+
+When the thread is no longer needed, the {{i: `nng_thread_destroy`}}
+function should be used to reap it.
+(This function will block waiting for _func_ to return.)
+
+> [!IMPORTANT]
+> Thread objects created by this function may not be real system-level
+> threads capable of performing blocking I/O operations using normal blocking system calls.
+> If use of blocking system calls is required (not including APIs provided
+> by the _NNG_ library itself of course), then real OS-specific threads
+> should be created instead (such as with `pthread_create` or similar functions.)
+
+> [!IMPORTANT]
+> Thread objects created by this function cannot be passed to any system threading functions.
+
+> [!TIP]
+> The system may impose limits on the number of threads that can be created.
+> Typically applications should not create more than a dozen of these.
+> If greater concurrency or scalability is needed, consider instead using
+> an asynchronous model using [`nng_aio`][aio] structures.
+
+> [!TIP]
+> Threads can be synchronized using [mutexes][mutex] and
+> [condition variables][condvar].
+
+In order to facilitate debugging, {{i:`nng_thread_set_name`}} may be called
+to provide a name for the thread. This may change how the thread is represented
+in debuggers. Not all platforms support setting the thread name.
+
+## RETURN VALUES
+
+The `nng_thread_create` function returns 0 on success, and non-zero otherwise.
+
+## ERRORS
+
+- `NNG_ENOMEM`: Insufficient free memory exists.
+
+## SEE ALSO
+
+[nng_cv][condvar],
+[nng_mutex][mutex]
+
+[condvar]: ../thr/nng_cv.md
+[mutex]: ../thr/nng_mtx.md
+[aio]: TODO.md