document .env.example with descriptions

.env.example

@@ -1,22 +1,69 @@
+# Port to listen on (binds to 127.0.0.1).
 BOOK_PORT=8123
+
+# Directory for guestbook entry files.
 BOOK_DATA_DIR=./data
+
+# Site title shown in nav and page title.
 BOOK_SITE_TITLE=guestbook
+
+# Base URL of the main site (for absolute nav links).
 BOOK_SITE_URL=https://example.com
+
+# Telegram bot token.
 BOOK_TELEGRAM_BOT_TOKEN=your-bot-token-here
+
+# Telegram chat ID for moderation messages.
 BOOK_TELEGRAM_CHAT_ID=0
+
+# Enable honeypot field for spam prevention.
 BOOK_HONEYPOT=true
+
+# Maximum length for names. 0 for unlimited.
 BOOK_MAX_NAME_LENGTH=50
+
+# Maximum length for messages. 0 for unlimited.
 BOOK_MAX_MESSAGE_LENGTH=1000
+
+# Maximum length for website URLs. 0 for unlimited.
 BOOK_MAX_WEBSITE_LENGTH=100
+
+# Allow new guestbook submissions. When false, the form is hidden and submissions are rejected.
 BOOK_OPEN_REGISTRATION=true
+
+# Separator between guestbook entries.
 BOOK_SEPARATOR=------------------------------------------------------------
-BOOK_FORM_PROMPT=If you visited my site, please sign my guestbook!
-BOOK_BUTTON_TEXT=sign
-BOOK_LABEL_NAME=Your name:
-BOOK_LABEL_WEBSITE=Your website (optional):
-BOOK_LABEL_MESSAGE=Your message:
-BOOK_TEXTAREA_ROWS=8
-BOOK_TEXTAREA_COLS=60
+
+# Path to a CSS file. Takes precedence over BOOK_STYLE.
 # BOOK_STYLE_FILE=./my-styles.css
-# BOOK_STYLE=.entry-name { font-weight: bold; }
+
+# Custom CSS injected into a style tag.
+# Classes: .guestbook-form, .guestbook-prompt, .guestbook-label, .guestbook-input,
+#          .guestbook-textarea, .guestbook-button, .entry-header, .entry-name,
+#          .entry-website, .entry-body, .entry-separator
+# BOOK_STYLE=
+
+# Text shown above the form.
+BOOK_FORM_PROMPT=If you visited my site, please sign my guestbook!
+
+# Submit button text.
+BOOK_BUTTON_TEXT=sign
+
+# Label for the name field.
+BOOK_LABEL_NAME=Your name:
+
+# Label for the website field.
+BOOK_LABEL_WEBSITE=Your website (optional):
+
+# Label for the message field.
+BOOK_LABEL_MESSAGE=Your message:
+
+# Number of rows for the message textarea.
+BOOK_TEXTAREA_ROWS=8
+
+# Number of columns for the message textarea.
+BOOK_TEXTAREA_COLS=60
+
+# Custom HTML template file with {{title}}, {{form}}, {{entries}}, and {{style}} placeholders.
+# Uses built-in default if unset.
 # BOOK_TEMPLATE=./templates/default.html