Document the namespaces and the opaque handles

dpm_core carries a namespace block saying what it holds and why its
members keep external linkage while staying off the export table. Each
unnamed namespace says its contents are private to that translation
unit, and the one in the info module says why it has to be unnamed
there: a module is built without hidden visibility or a version script,
so a helper with external linkage would reach that module's exports.

The three opaque handles in the public header were undocumented. What
documentation they had came from the struct definitions in
include/internal, which a consumer never sees.

EXTRACT_ANON_NSPACES was off, so the documentation on every file-private
helper was written and discarded. Doxygen also capitalizes the first
letter of a brief, which turned libdpm-core.so into Libdpm-core.so
wherever a brief opened with it; those are reworded.

Bare references to the library are replaced with the artifact name
throughout, including three in the info module's output. The
cli_info_version regex follows.

docs/CMakeLists.txt

@@ -15,6 +15,11 @@ if(DOXYGEN_FOUND)
     set(DOXYGEN_OUTPUT_DIRECTORY ${DPM_DOCS_TMP_DIR})
     set(DOXYGEN_EXTRACT_ALL YES)
     set(DOXYGEN_EXTRACT_STATIC YES)
+
+    # Helpers private to a translation unit live in unnamed namespaces,
+    # whose contents Doxygen drops unless this is on. Without it their
+    # documentation is written and never reaches the output.
+    set(DOXYGEN_EXTRACT_ANON_NSPACES YES)
     set(DOXYGEN_QUIET YES)
     set(DOXYGEN_WARN_IF_UNDOCUMENTED NO)
     set(DOXYGEN_JAVADOC_AUTOBRIEF YES)