Improve sst_dump help message

Summary:
Current Message

```
sst_dump [--command=check|scan|none|raw] [--verify_checksum] --file=data_dir_OR_sst_file [--output_hex] [--input_key_hex] [--from=<user_key>] [--to=<user_key>] [--read_num=NUM] [--show_properties] [--show_compression_sizes] [--show_compression_sizes [--set_block_size=<block_size>]]
```
New message

```
sst_dump --file=<data_dir_OR_sst_file> [--command=check|scan|raw]
    --file=<data_dir_OR_sst_file>
      Path to SST file or directory containing SST files

    --command=check|scan|raw
        check: Iterate over entries in files but dont print anything except if an error is encounterd (default command)
        scan: Iterate over entries in files and print them to screen
        raw: Dump all the table contents to <file_name>_dump.txt

    --output_hex
      Can be combined with scan command to print the keys and values in Hex

    --from=<user_key>
      Key to start reading from when executing check|scan

    --to=<user_key>
      Key to stop reading at when executing check|scan

    --read_num=<num>
      Maximum number of entries to read when executing check|scan

    --verify_checksum
      Verify file checksum when executing check|scan

    --input_key_hex
      Can be combined with --from and --to to indicate that these values are encoded in Hex

    --show_properties
      Print table properties after iterating over the file

    --show_compression_sizes
      Independent command that will recreate the SST file using 16K block size with different
      compressions and report the size of the file using such compression

    --set_block_size=<block_size>
      Can be combined with --show_compression_sizes to set the block size that will be used
      when trying different compression algorithms
```

Test Plan: none

Reviewers: yhchiang, andrewkr, kradhakrishnan, yiwu, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56325

tools/sst_dump_tool.cc

@@ -348,16 +348,44 @@ namespace {
 
 void print_help() {
   fprintf(stderr,
-          "sst_dump [--command=check|scan|none|raw] [--verify_checksum] "
+          R"(sst_dump --file=<data_dir_OR_sst_file> [--command=check|scan|raw]
-          "--file=data_dir_OR_sst_file"
+    --file=<data_dir_OR_sst_file>
-          " [--output_hex]"
+      Path to SST file or directory containing SST files
-          " [--input_key_hex]"
+
-          " [--from=<user_key>]"
+    --command=check|scan|raw
-          " [--to=<user_key>]"
+        check: Iterate over entries in files but dont print anything except if an error is encounterd (default command)
-          " [--read_num=NUM]"
+        scan: Iterate over entries in files and print them to screen
-          " [--show_properties]"
+        raw: Dump all the table contents to <file_name>_dump.txt
-          " [--show_compression_sizes]"
+
-          " [--show_compression_sizes [--set_block_size=<block_size>]]\n");
+    --output_hex
+      Can be combined with scan command to print the keys and values in Hex
+
+    --from=<user_key>
+      Key to start reading from when executing check|scan
+
+    --to=<user_key>
+      Key to stop reading at when executing check|scan
+
+    --read_num=<num>
+      Maximum number of entries to read when executing check|scan
+
+    --verify_checksum
+      Verify file checksum when executing check|scan
+
+    --input_key_hex
+      Can be combined with --from and --to to indicate that these values are encoded in Hex
+
+    --show_properties
+      Print table properties after iterating over the file
+
+    --show_compression_sizes
+      Independent command that will recreate the SST file using 16K block size with different
+      compressions and report the size of the file using such compression
+
+    --set_block_size=<block_size>
+      Can be combined with --show_compression_sizes to set the block size that will be used
+      when trying different compression algorithms
+)");
 }
 
 }  // namespace