Note that white space on either side of the comments can end up in the output stream, depending on your XSLT processor and its settings for handling white-space. If this is an issue for your output, make sure the comment is bracketed by xslt tags.
EG
<xsl:for-each select="someTag">
<xsl:text>"</xsl:text>
<!-- output the id -->
<xsl:value-of select="@id"/>
<xsl:text>"</xsl:text>
</xsl:for-each>
Will output " someTagID" (the indent tab/spaces in front of the comment tag are output).
To remove, either unindent it flush with left margin, or bracket it like
<xsl:text>"</xsl:text><!-- output the id --><xsl:value-of select="@id"/>