Add placeholder text for "Add SSH/GPG Key" forms (#12533)

* Add placeholder text for "Add SSH/GPG Key" forms

This commit add placeholder text for both Add SSH key and Add GPG key
forms.

* Localize placeholders

Changes requested by @zeripath

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

options/locale/locale_en-US.ini

@@ -500,6 +500,8 @@ ssh_helper = <strong>Need help?</strong> Have a look at GitHub's guide to <a hre
 gpg_helper = <strong>Need help?</strong> Have a look at GitHub's guide <a href="%s">about GPG</a>.
 add_new_key = Add SSH Key
 add_new_gpg_key = Add GPG Key
+key_content_ssh_placeholder = Begins with 'ssh-ed25519', 'ssh-rsa', 'ecdsa-sha2-nistp256', 'ecdsa-sha2-nistp384', or 'ecdsa-sha2-nistp521'
+key_content_gpg_placeholder = Begins with '-----BEGIN PGP PUBLIC KEY BLOCK-----'
 ssh_key_been_used = This SSH key has already been added to the server.
 ssh_key_name_used = An SSH key with same name is already added to your account.
 gpg_key_id_used = A public GPG key with same ID already exists.

templates/user/settings/keys_gpg.tmpl

@@ -47,7 +47,7 @@
 			<input type="hidden" name="title" value="none">
 			<div class="field {{if .Err_Content}}error{{end}}">
 				<label for="content">{{.i18n.Tr "settings.key_content"}}</label>
-				<textarea id="gpg-key-content" name="content" required>{{.content}}</textarea>
+				<textarea id="gpg-key-content" name="content" placeholder="{{.i18n.Tr "settings.key_content_gpg_placeholder"}}" required>{{.content}}</textarea>
 			</div>
 			<input name="type" type="hidden" value="gpg">
 			<button class="ui green button">

templates/user/settings/keys_ssh.tmpl

@@ -51,7 +51,7 @@
 			</div>
 			<div class="field {{if .Err_Content}}error{{end}}">
 				<label for="content">{{.i18n.Tr "settings.key_content"}}</label>
-				<textarea id="ssh-key-content" name="content" required>{{.content}}</textarea>
+				<textarea id="ssh-key-content" name="content" placeholder="{{.i18n.Tr "settings.key_content_ssh_placeholder"}}" required>{{.content}}</textarea>
 			</div>
 			<input name="type" type="hidden" value="ssh">
 			<button class="ui green button">