MDEV-20672 Inconsistent usage message for innodb_compression_algorithm

The usage message for the innodb_compression_algorithm system variable
did not list snappy, which was added as an optional compression algorithm
in MariaDB 10.1.3 and might actually work since
commit 90c52e52 (MDEV-12615)
in MariaDB 10.1.24.

Unfortunately, we will include also unavailable compression algorithms
in the list, because ENUM parameters allow numeric values, and we do
not want innodb_compression_algorithm=3 to change meaning depending on
the way how the source code was compiled.

mysql-test/suite/sys_vars/r/sysvars_innodb.result

@@ -502,7 +502,7 @@ GLOBAL_VALUE_ORIGIN	COMPILE-TIME
 DEFAULT_VALUE	zlib
 VARIABLE_SCOPE	GLOBAL
 VARIABLE_TYPE	ENUM
-VARIABLE_COMMENT	Compression algorithm used on page compression. One of: none, zlib, lz4, lzo, lzma, or bzip2
+VARIABLE_COMMENT	Compression algorithm used on page compression. One of: none, zlib, lz4, lzo, lzma, bzip2, or snappy
 NUMERIC_MIN_VALUE	NULL
 NUMERIC_MAX_VALUE	NULL
 NUMERIC_BLOCK_SIZE	NULL

storage/innobase/handler/ha_innodb.cc

@@ -19862,7 +19862,7 @@ static TYPELIB page_compression_algorithms_typelib=
 };
 static MYSQL_SYSVAR_ENUM(compression_algorithm, innodb_compression_algorithm,
   PLUGIN_VAR_OPCMDARG,
-  "Compression algorithm used on page compression. One of: none, zlib, lz4, lzo, lzma, or bzip2",
+  "Compression algorithm used on page compression. One of: none, zlib, lz4, lzo, lzma, bzip2, or snappy",
   innodb_compression_algorithm_validate, NULL,
   /* We use here the largest number of supported compression method to
   enable all those methods that are available. Availability of compression

storage/xtradb/handler/ha_innodb.cc

@@ -21141,7 +21141,7 @@ static TYPELIB page_compression_algorithms_typelib=
 };
 static MYSQL_SYSVAR_ENUM(compression_algorithm, innodb_compression_algorithm,
   PLUGIN_VAR_OPCMDARG,
-  "Compression algorithm used on page compression. One of: none, zlib, lz4, lzo, lzma, or bzip2",
+  "Compression algorithm used on page compression. One of: none, zlib, lz4, lzo, lzma, bzip2, or snappy",
   innodb_compression_algorithm_validate, NULL,
   /* We use here the largest number of supported compression method to
   enable all those methods that are available. Availability of compression