Update documentation

Author: franzpoeschel

src/cli/merge-json.cpp

@@ -2,13 +2,17 @@
 
 void print_help_message(char const *program_name)
 {
-    std::cout << "Usage: " << std::string(program_name) << R"( [json_or_toml]+
+    std::cout << "Merge multiple JSON/TOML files into one.\nUsage: "
+              << std::string(program_name) << R"( [json_or_toml]+
 'json_or_toml' can be a JSON or TOML dataset specified inline or a reference
 to a file prepended by an '@'.
 Inline datasets will be interpreted as JSON if they start with an '{', as TOML
 otherwise. Datasets from a file will be interpreted as JSON or TOML depending
 on the file ending '.json' or '.toml' respectively.
-Inline dataset specifications can be replaced by input read from stdin.
+
+In order to support large numbers of files to be merged, the paths to those
+files can also be specified line-by-line per stdin, replacing the limitations
+of command line arguments.
 
 If the JSON/TOML files are mixed, then the output type (JSON or TOML) will be
 determined by the type of the first file.