mdbx: Merge branch 'master' into c++.

Change-Id: I7431973ca96afe98d991ffd0a876a15e6ed94714

GNUmakefile

@@ -274,7 +274,7 @@ docs/intro.md: docs/_preface.md docs/__characteristics.md docs/__improvements.md
 	cat $^ | sed 's/^Performance comparison$$/Performance comparison {#performance}/' > $@
 
 docs/usage.md: docs/__usage.md docs/_starting.md docs/__bindings.md
-	echo -e "\\page usage Usage\n\\section getting Getting the libmdbx" | cat - $^ | sed 's/^Bindings$$/Bindings {#bindings}/' > $@
+	echo -e "\\page usage Usage\n\\section getting Building & Embedding" | cat - $^ | sed 's/^Bindings$$/Bindings {#bindings}/' > $@
 
 doxygen: docs/Doxyfile docs/overall.md docs/intro.md docs/usage.md mdbx.h mdbx.h++ src/options.h ChangeLog.md AUTHORS LICENSE
 	rm -rf docs/html && cp mdbx.h mdbx.h++ src/options.h ChangeLog.md docs/ && (cd docs && doxygen Doxyfile) && cp AUTHORS LICENSE docs/html/

README.md

@@ -3,6 +3,11 @@
 libmdbx
 ========
 
+> Please refer to the online [documentation](https://erthink.github.io/libmdbx/)
+> with [`C` API description](https://erthink.github.io/libmdbx/group__c__api.html)
+> and pay attention to the preliminary [`C++` API](https://github.com/erthink/libmdbx/blob/c%2B%2B/mdbx.h%2B%2B).
+> Questions, feedback and suggestions are welcome to the [Telergam' group](https://t.me/libmdbx).
+
 <!-- section-begin overview -->
 _libmdbx_ is an extremely fast, compact, powerful, embedded,
 transactional [key-value database](https://en.wikipedia.org/wiki/Key-value_database),

docs/_toc.md

@@ -8,30 +8,11 @@ each of which is divided into several sections.
 
 1. The \ref intro
   - \ref characteristics
-    - Preface
-    - Features
-    - Limitations
-    - Gotchas
-    - Comparison with other databases
   - \ref restrictions
   - \ref performance
-    - Integral performance
-    - Read Scalability
-    - Sync-write mode
-    - Lazy-write mode
-    - Async-write mode
-    - Cost comparison
 2. \ref usage
   - \ref getting
-    - Embedding
-    - Building
   - \ref starting
-    - Opening
-    - Cursors
-    - Threads and processes
-    - Transactions
-    - Duplicate keys aka Multi-values
-    - Cleaning up
   - \ref bindings
 
 3. The `C/C++` API manual:

mdbx.h

@@ -1070,15 +1070,20 @@ enum MDBX_env_flags_t {
    * be written to disk, while the itself B-tree not yet. In that case, the
    * database will be corrupted!
    *
-   * \see MDBX_NOMETASYNC \see MDBX_SAFE_NOSYNC \see MDBX_UTTERLY_NOSYNC
+   * \see MDBX_SYNC_DURABLE \see MDBX_NOMETASYNC \see MDBX_SAFE_NOSYNC
+   * \see MDBX_UTTERLY_NOSYNC
    *
    * @{ */
 
   /** Default robust and durable sync mode.
    *
-   * Metadata is written and flushed to disk after data is written and flushed,
-   * which guarantees the integrity of the database in the event of a crash at
-   * any time. */
+   * Metadata is written and flushed to disk after a data is written and
+   * flushed, which guarantees the integrity of the database in the event
+   * of a crash at any time.
+   *
+   * \attention Please do not use other modes until you have studied all the
+   * details and are sure. Otherwise, you may lose your users' data, as happens
+   * in [Miranda NG](https://www.miranda-ng.org/) messenger. */
   MDBX_SYNC_DURABLE = 0,
 
   /** Don't sync the meta-page after commit.