rust: Document `pspp decrypt`.

diff --git a/rust/doc/src/invoking/pspp-decrypt.md b/rust/doc/src/invoking/pspp-decrypt.md
index 1f8b5447df5ea4146fffe8ea7d5be2abc1fbb7b1..8d17c020e62218c9b4c66ef2150f40ac052b7e75 100644 (file)
--- a/rust/doc/src/invoking/pspp-decrypt.md
+++ b/rust/doc/src/invoking/pspp-decrypt.md
@@ -1 +1,27 @@
 # Decrypting SPSS files with `pspp decrypt`
+
+SPSS supports encryption using a password for data, viewer, and syntax
+files.  `pspp decrypt <INPUT> <OUTPUT>` reads an encrypted file
+`<INPUT>` and writes an equivalent plaintext file `<OUTPUT>`.
+
+Other commands, such as [`pspp convert`](pspp-convert.md), can also
+read encrypted files directly.
+
+PSPP does not support writing encrypted files, only reading them.
+
+> ⚠️ Warning: The SPSS encryption format is insecure: when the password
+> is unknown, it is much cheaper and faster to decrypt a file
+> encrypted this way than if a well designed alternative were used.
+
+## Options
+
+`pspp decrypt` accepts the following options:
+
+* `-p <PASSWORD>`  
+  `--password <PASSWORD>`  
+  Specifies the password for reading the encrypted input file.
+  Without this option, PSPP will interactively prompt for the
+  password.
+
+  > ⚠️ The password (and other command-line options) may be visible to
+  other users on multiuser systems.